The goal of this project is to restore the glory of the legendary Aerosoft Twin Otter X Extended (by Finn Jacobsen & team, for FSX and P3D) using the modern systems available in MSFS2024, based on the Aerosoft Twin Otter for MSFS2020 (by Hans Hartmann & team). This project is not meant to compete with Microsoft’s Twin Otter in MSFS2024 (by Hans Hartmann & a different team), which is built for easy accessibility.

Outside of the scope of this project:

• 3d model
• sound, at least for the time being, because of utter lack of expertise

Inside the scope:

• everything that can be verified without input from actual pilots (until actual pilots chime in)
• quality-of-life improvements
• improvements to realism (inconsistently overridden by quality-of-life)
• damage (WIP)
• weird things I want to try out or learn about

You know how you have your best flight simming moments that you will (hopefully) never forget and that, after years and years, still fill you with joy when you recall them? One of those moments for me was my first hot start of the Aerosoft Twin Otter X Extended in FSX or P3D, about a decade ago. That was the day I learned to treat my engines with respect. I started actively looking for aircraft that could break if treated badly, discovered RealAir and A2A, and learned to appreciate deeper simulation.

Speaking of A2A, another one of those moments was the discovery that it felt completely different (to me) when an aircraft had state. Simple things like the avionics remembering what frequencies I had used last, switches I had left in this or that position, they created a sense of ownership I really appreciated.

So, yeah, the response to “Why?” is simply “Nostalgia!”

NOTE: Because of previous drama around Aerosoft mods, this mod is distributed as an archive containing xdelta files and an install script. You will need an original, unmodded installation of the Twin Otter 1.1.1 (installed using Aerosoft One, the legacy Aerosoft installer, or by other means) so the installation process can READ the original files and WRITE the changed files to the mod folder. Your original files will not be touched.

Until the install script has been executed, the mod will be inactive.

Before you report an installation error, please consider reinstalling a fresh copy of the Aerosoft Twin Otter in the last release version (1.1.1) and trying with that folder (funny story: I spent an hour trying to find an imaginary bug in the install script because I didn’t remember that I tweaked my Twotter years ago by changing a few settings in a .flt file). If the errors persist, please contact me so we can figure out why they happen. The installer should now work with all popular versions of 1.1.1.

• unpack directly into community or wherever you keep your Community add-ons (the zip contains the top folder of the mod: charliebravo-aircraftmod-twin-otter and an optional prop blur mod by CCM: z_aerosoft-twin-otter_propblur_mod)
• open the folder charliebravo-aircraftmod-twin-otter
• optionally, if you’re paranoid or they are actually after you: examine install.bat and install.ps1, overwrite the xdelta executable with a version you trust
• double-click install.bat or run install.ps1 in Powershell (install.bat is just a wrapper that executes the Powershell script)
• follow the prompts and select an unmodified aerosoft-aircraft-twin-otter folder (only for reference, nothing in that folder will be written or changed)

In a perfect world, the changes are applied and you end up with a working mod and the lawyers have no leverage against me. I apologize for this, I can assure you that it has been even less fun to set this up. I contacted Aerosoft and Microsoft to get their blessing, but, so far, I didn’t receive a response. It appears our Twotter is in some legal limbo: Aerosoft sold the rights to Microsoft, and now neither party appears to have any interest in the legacy product. xdelta helps me to stay on the safe side.

You will still need to have the base model (ie. the aerosoft-aircraft-twin-otter folder) in your Community/ folder for the mod to work.

Liveries

Legacy liveries for the Aerosoft model might work, if the loading order is carefully managed. You might want to rename charliebravo-aircraftmod-twin-otter to zzz-charliebravo-aircraftmod-twin-otter to ensure the mod is loaded after base aircraft and liveries.

• Ways to identify whether the mod is loaded: 1) check the “More Apps” section of the standard EFB, it should have a “Twin Otter” app whenever the Twotter is loaded, or 2) load up cold & dark, turn on the power and check the GNS units. The new WT versions go through a lengthy initialization cycle (mod is active), while the old Asobo versions are immediately on a nav screen (mod not active).
• If you want persistence in your cockpit you SHOULD start cold & dark and you MUST click the “Initialize persistence” or “Restore state” button in the EFB app at the beginning of your flight (or at least while on the ground).
• Engine startup: If you follow the checklists, you should be good. (If you follow the checklists and something doesn’t work as the checklists say, pls let me know.)
• Engine startup (advanced): If you follow the usual PT6A startup procedure, you should be good.
• Engine startup (nostalgic): If you want to experience a hot start, push the condition lever before the starter has stabilized. The earlier, the more fun! (There’s no damage modelling yet, so no worries, you can go flying anyway.)

The list below is the v2024.3.124 state. New entries since v2024.3.99 are tagged [NEW]; entries whose behavior changed materially are tagged [CHANGED].

EFB “Twin Otter” app

• Added EFB app with Load Manager for pax and cargo variants.
• Load manager: short click on a seat or cargo station toggles between 0 and the set weight, long click resets to max weight.
• Sliders allow for quick adjustment of passenger, cargo and fuel loads. All changes take effect immediately.
• Added “Redist Fuel” button to move fuel from wing tips to main FWD/AFT tanks ().
• Added Info and Debug tabs to the EFB.
• Added a Stats tab in lieu of the missing Hobbs meters.
• Added a Doors tab to the EFB for control of all aircraft cabin and cargo doors.
• Added a Systems tab for chocks and the hydraulic emergency hand pump.
• Added an Aircraft tab with battery temperature, bleed air pressure, and other system readouts.
• Added a Flight tab with environmental info and ice-accretion statistics.
• Added a Maintenance tab for the Wear & Tear system.
• Added a TAWS tab for debugging and educational purposes.
• Added a Radio tab for debugging and educational purposes.
• Added a Config tab.
• The splash screen with the “Restore state” button only appears if the engines aren’t running. You can still restore the state using the button on the debug tab.
• The splash opens in a “Waiting for flight to start” state with no action buttons; Restore State / Continue-without-persistence appear only once the flight is ready.

Configurable settings

The default settings are either sim defaults or they enable convenience features.
The non-default settings strive to be more realistic, and frequently end up being more annoying.

Each option has an override checkbox on the left: leave it unchecked and the setting is shared across all variants that don’t override it; check it and that variant keeps its own value, independent of the rest.

• Disable Autopilot Auto-ARM
  default: a selected altitude will be captured automatically
  when turned on: a selected altitude will only be captured if “ARM” has been pressed
• Disable Flight Director
  default: the pitch and roll command bars are drawn on the attitude indicator
  when turned on: the command bars are hidden
• Realistic battery capacity
  default: a phantom charger will keep your batteries topped up forever
  when turned on: batteries will drain based on the enabled consumers; if you drain the battery, either switch the chemistry or use the virtual GPU by setting the source switch to “EXT”
• Battery Replacement
  default: standard NiCd pack
  when turned on: battery is replaced with a SLA pack with higher capacity, but also higher weight
• Realistic parking brake
  default: standard sim parking brake
  when turned on: to engage the parking brake, you will have to depress both toe brakes fully and THEN pull the handle (or toggle the parking brake with an input event). If the latching mechanism catches the toe brakes, the parking brake handle stays out. If it snaps back, you have to try again.
• Generators can trip
  default: generators can’t trip
  when turned on: generators can trip on under- or overvoltage, the affected generator will need to be RESET to be reconnected to the bus
• Gyros take time to spin up
  default: AI and HSI are available immediately when power is provided
  when turned on: gyros have to spin up and the gauges can take a significant amount of time to show correct values; the “Pull to Erect” knob significantly shortens the time for the attitude indicator; the HSI can be synced to the whiskey compass using a button on the Systems tab of the EFB.
• Disable Auto Fast-Erect
  default: with realistic gyros, the attitude indicators will magically pull the fast erect knob
  when turned on: unless you use the fast erect, it will take a significant time for the horizon to level
• Disable OBS Auto-Slew
  default: the GNS530 can control the CDI needle of the HSI
  when turned on: you have to set the CDI needle yourself
• Reliability (slider: Off / Reliable / Unreliable / Not Airworthy)
  default: Off — the Wear & Tear system is disabled and the Maintenance tab is hidden
  Reliable: nominal wear rate (“real-time”), experienced mechanic, generous (risk-averse) management
  Unreliable: 4× wear rate
  Not Airworthy: 16× wear rate, inexperienced mechanic working for a rinky-dink outfit with limited access to new parts
• TAWS (selector: Full / Typical / Legal minimum / Class B / Off)
  Full: every callout on every variant — terrain, sink rate, glideslope, bank angle, the full altitude-callout ladder, and MINIMUMS.
  Typical: passenger variants get the full set; cargo/utility variants get the reduced Class B set.
  Legal minimum: passenger variants only; cargo/utility variants get nothing.
  Class B: the reduced TSO-C151 Class B set everywhere — excessive descent rate, altitude loss after takeoff, the “500 feet” callout, and MINIMUMS (no terrain, glideslope, or bank-angle callouts).
  Off: all callouts muted.
• The TAWS section gained settings to select the soundbank and set the volume.
• [NEW] Disable relaxed NAV arming: hands NAV arming straight to the GPS/NAV autopilot.
• [NEW] Enable historic NDBs: adds decommissioned/historical NDB beacons that modern navdata dropped.

Checklists

• Autocomplete: copilot runs and verifies each item, with spoken confirmations.
• Interactive: pilot acts, copilot verifies.
• Copilot callouts: Several dozens responses, most with alternate takes so he doesn’t sound like a robot. He announces “Checklist complete!” when a page finishes on its own. Our research showed that a soothing copilot voice can help to reduce crew stress and prevent accidents, so we picked the most soothing voice we could find. All callouts are plain wav files.
• “As required” items read the conditions: a few items work out the right setting from the world around you instead of forcing a fixed state (for example: Anti-Ice, Taxi Light).
• In Autocomplete, a page that commands the engines or fuel will ask for confirmation first.
• Climb Power and Cruise Power wait for you: there’s no single torque or ITT number a copilot could verify for “climb power” or “cruise power”. Even in Autocomplete he stops on these items and waits for you to set the power and tick them off. The copilot won’t touch flight controls or power settings while handling in-flight checklists.

Cockpit/aircraft persistence

• The state saving is mostly opt-in. (Only a few select states persist unconditionally, e.g. engine block temperatures, cosmetic airframe variations.)
• First time: while on the ground and, preferably, cold & dark, use the “Enable Persistence” button on the splashscreen of the EFB (or, if you missed it, the “Restore State” button on the Debug tab).
• Cockpit state, fuel and payload can be restored from the EFB app (“Restore State”). Previously saved values will not be overwritten or updated unless “Restore State” is used. This is intended to allow ad-hoc flights that don’t affect the saved state.
• To reset the saved state, locate state.cfg for the variant in your MSFS folder and delete or rename it.
• Persisted items now include the BARO setting, decision height, transponder code, all active and standby COM/NAV frequencies, the heading bug, ITT block residual heat, and battery temperatures.
• Persistence also covers the radio audio panel (transmit and receive selectors).
• Config settings can now be saved fleet-wide or per variant: each option on the EFB Config tab has an override checkbox — leave it unchecked and the value is shared across your whole Twin Otter fleet, check it and that variant keeps its own value.
• Known issues: 1) Some systems don’t persist (battery capacity, hydraulic system state). 2) “Restore state” does not prevent you from restoring the state with running engines, but you probably shouldn’t.

Flight model

Captain Moustache writes:

What I’ve done:

• decreased nosewheel steering sensitivity at medium to high speeds
• adjusted main gear springiness to stop the rolling back and forth and make it sit slightly nose-low on the ground
• drastically increased wing incidence angle
• removed the invisible spoiler
• added prop drag
• decreased elevator and rudder sensitivity around neutral
• increased fuselage side-area
• added ground friction and crosswind numbers
• increased icing physical impact to 400%
• decreased overall rudder maximum effectiveness
• increased elevator trim effectiveness
• decreased gyro stability around all axes
• doubled flap lift
• added 40% to flap drag
• decreased blade angle on the ground to minimize aircraft rolling at idle and quasi-simulate beta range
  This leads to a Twotter that looks and handles much more like the real thing. Expect that iconic DHC nose-down on approach. Expect massive amounts of both lift and drag beyond 20 degrees of flap. The plane will also handle far better on the ground, both in the taxi and on takeoff. I’m by no means claiming this is the be-all, perfect flight model for this plane and it will likely evolve some but for now we’re hitting book numbers in most cases within 1-2% and the plane is a sheer joy to fly.

• CCM tuning is now the default for every base variant: a ground-up flight-model, flaps, prop, and steering overhaul by CanadianCaptainMoustache, pushing toward realism and giving the Twotter its nose-down “stance” in cruise and descent.
• Beta-range annunciators rewired to harmonize with CCM’s prop/throttle changes.
• Refined propeller lift characteristics.
• Restored P-factor effect on yaw.
• Changed slip indicator to be more sensitive.
• Retired the spoiler-based disking hack. Under CCM (the shipped tuning) disking falls out of the flight physics directly, so the spoiler proxy is gone — matching the real DHC-6, which has no spoilers.
• Fixed too many entries in control surface elasticity tables (MSFS2024 complained).
• New propeller blur — CCM’s prop-blur textures are bundled in the archive as a separate mod. <3 CCM <3

Suspension

• Adjusted nosewheel suspension damping.
• Adjusted ground contact model parameters and gear flex damping.
• Tuned Tundra Pax based on the tundra wheel variant of the Aerosoft Twin Otter X Extended.

Avionics

• Updated GNS530/430 to WT530/430. Wired display brightness to the ambient light sensor.
• Radio frequencies and CDI selection are saved if persistence is enabled.
• Slaved HSI to GNS530 (with config option).
• Took ownership of the WT GNS 530/430 gauges; GNS range and page state now persist.

Radio navigation

The goal is: replace all navaid signals with a physics- and terrain-based simulation.
State: Mostly complete. The custom signal now drives the steam gauges and the autopilot directors.

All radio navaid signals are now simulated in custom code. All of the needles, TAWS and AP are driven by signal-strength-first propagation models. Stations fade, drop out behind ridges, interfere with each other, and the needles wander, jitter and lag. And there’s an EFB tab to show you why.

Note: At this point, ONLY the steam gauges are migrated to the new system. AP and GNS units are still using stock sim data (but not for very much longer). If the needles show a slight deviation while you’re on AP, that’s expected. If the needles do one thing and the AP does a totally different thing, that’s possibly a bug that should be reported, ideally with video or screenshots of the Radio tab.

UHF/VHF: VOR, ILS localizer & glideslope, DME (line-of-sight bands):

• Free-space path loss: inverse-square spreading on the true slant range (Friis free-space formula).
• Terrain diffraction & shadowing: the Longley-Rice Irregular Terrain Model (the NTIA/ITS ITM), run against a baked global terrain and land-cover dataset (≈1.85 km cells, derived from ESA WorldCover land cover and Natural Earth water/ice).
• DME on its real frequency: DME is paired to its true UHF / L-band channel (ICAO Annex 10) and run through a separate terrain pass, so it digs a deeper shadow behind a ridge than the co-located VOR.
• Service volume & antenna pattern: per-class range and elevation/cone roll-off curves (ICAO service volumes; libradio curves).
• Co-channel interference (12 dB capture): of all stations on the tuned frequency, the receiver locks only the strongest, and only if it leads the runner-up by ≥ 12 dB; inside that margin it garbles to a dropout (ICAO capture ratio).
• Radial & CDI geometry: the VOR radial and course deviation are built from the station’s position, its commissioned magnetic variation, and your OBS course (ICAO Annex 10).
• Per-runway localizer width: the on-course sector width is derived from the actual runway geometry, clamped to the Annex-10 bounds.
• False glideslopes: capture lobes at odd multiples of the true GS angle, falling out of the antenna geometry (ICAO Annex 10).
• Independent glideslope signal: GS strength is computed separately from the localizer.
• Signal-dependent needle lag: the weaker the signal, the heavier the servo smoothing and the more the needle lags (libradio lag law).
• Usable / identifiable floor (hysteretic): usable at −70 dB, can’t-ID below −73 dB; below the floor the CDI parks to centre, the GS retracts, TO/FROM reads OFF and the NAV flag drops (libradio dB thresholds).
• DME receiver behaviour: slant range, an overhead no-track zone, memory/hold on signal loss, and a line-of-sight horizon range limit (modelled on the KN-62).

LF/MF: NDB (ground-wave + night sky-wave):

• Ground-wave field strength: ITU-R Recommendation P.368 ground-wave curves (1 kW vertical-monopole reference) over detailed ground regimes based on nine terrain types plus permafrost and wet tundra.
• Mixed land–sea paths: Millington’s method (P.368 Annex 2); surface conductivity comes from the baked land-cover/water dataset.
• Transmit power by class: NDB power scaled per service class.
• Height gain: an elevated-receiver correction, since the surface-only P.368 curves under-read for an aircraft at altitude.
• LF terrain masking: a gentle knife-edge loss (3–15 dB, vs VHF’s 20–60), because long LF wavelengths bend around terrain.
• Night sky-wave range extension: ITU-R Recommendation P.1147 single-hop E-layer night field, power-summed with the ground wave.
• Diurnal variation: the P.1147 hourly absorption factor: sky-wave is crushed by day and deepest in the middle of the night.
• Seasonal variation: a P.1147-style seasonal term (MF semi-annual / LF annual, scaled by frequency and latitude, hemisphere-aware).
• Geomagnetic-latitude absorption: the dominant night-field absorption scales with geomagnetic latitude (centered-dipole model).
• Coastal refraction: bearings bend where the signal path crosses a coastline at a shallow angle (P.368 coastal effect).
• Quadrantal error: airframe re-radiation distorts the bearing as a per-airframe sine series (FAA Order 6740.6).
• Bank / dip error: falls out of a full 3-D line-to-station projection: a bank tilts the loop plane and the needle errs, worst at the fore/aft headings.
• NDB cone of silence: a narrow overhead cone (~0.2 NM radius at 10,000 ft) where the needle wanders.
• Night-effect bearing swing: twilight ionospheric fluctuation, gated by solar elevation, that makes the needle swing around dawn and dusk.
• Near-field multipath: a slowly-rotating reflection ghost whose near-field zone scales with the tuned wavelength.
• [NEW] Deterministic, signal-driven bearing error.
• [NEW] Curated historical NDB database (~9k beacons) with baked class/range and first/last-seen years.
• [NEW] Offline NDB classification: station class/range derived from location + terrain heuristics.
• [NEW] Terrain relief & urban bending, coastal-refraction overhaul, noise floor varying with geography/time/season, and co-channel stations combining as a field-weighted resultant bearing.

EGPWS

• Overhauled EGPWS suite based on Honeywell Pilot’s Guide and FAA advisories.
• EGPWS callouts are now configurable via the TAWS selector on the EFB Config tab.
• EPGWS callouts come in selectable soundbanks (Aerosoft, FlyByWire, Custom) with varying coverage. Only Custom has the complete set of callouts.
• Declutter governor: an advisory is only repeated if the situation degrades.
• The GPWS INHIBIT button suppresses some callouts and desensitizes others.
• The EFB has a TAWS tab that shows per-mode margin bars and Honeywell-style envelope plots.
• [NEW] EGPWS terrain warnings are now driven by the custom radio-signal / propagation model.

KAP 140 autopilot (custom gauge)

The autopilot is one of the areas where I’m claiming artistic license. While I fixed a couple of “realism gaps” with the default KAP 140, I also created new ones. Aerosoft put the KAP 140 in the Twotter for two reasons, afaik: users are familiar with it and the mission statement for the 2020 version was to use default avionics wherever possible. The visual model has all the remnants of the Collins AP-106 (I think) simulated in the Aerosoft Twin Otter X Extended: the annunciators, the mode and altitude panels, the multifunction “knob” on the control column pedestal, the YD button. The 2020 Twotter’s AP is already an unholy abomination, and I leaned into that.

The main realism features are:

• capture only when armed (this was a bitch to get right);
• “Vertical Trim”: tapping UP/DN nudges the aircraft in steps of 20 ft (another bitch) and
• holding UP/DN initiates a climb/descend at 500 fpm that is maintained until the button is released (yeah, a bitch, as well);
• obnoxiously long pre-flight test followed by
• a 30s pitch lockout (a red P on display replaces the dedicated warning light which is missing from our visual model).

The main realism gaps are:

• from the Collins AP, we inherit support for IAS/FLC mode, yaw damper, flight director,
• mode buttons and altitude selector are synced with the KAP 140 and
• the Collins annunciators are somehow magically driven by the KAP 140.

Known issues: underneath, we use the sim’s AP with all its quirks. The AP still uses the Captain’s HSI baro instead of its own.

• Altitude alerter runs a 3-second self-test.
• Pitch-trim arrow indicator is now a runaway-trim warning indicator (flashes for end-of-travel and same-direction-≥2 s commands).
• Enabled autopilot annunciators for approach arm/capture when using GPS navigation.
• Three-row LCD layout matching the physical hardware. PFT timing matches a reference video (≈40 s).
• Preflight test plays a completion chime when it ends.
• Daylight-aware unpowered LCD rendering: the display shows the dim ghost segments of an unlit LCD.
• Yaw-Damper (YD) annunciator support.
• Flashing AP-master annunciator with a 5-second flash on disconnect; cavalry-charge tone wired to the disconnect event.
• Held UP/DN ramps the vertical reference; short presses “nudge” by 20 ft (ALT mode), 100 fpm (VS mode) or 5 kts (FLC).
• FLC mode displays the selected airspeed; using UP/DN snaps the reference to the nearest 5 kt boundary.
• Sticky BARO reminder flashes after the preflight test until the pilot acknowledges by pressing BARO or changing the setting.
• Pitch-axis lockout (blinking “P” indicator) for 30 seconds after the preflight test and after every in-flight power cycle, while the accelerometer settles.
• G-load monitor auto-disengages the autopilot if the pilot pulls or pushes through a threshold.
• Experimental: NAV “pre-arm” allows arming NAV when the sim AP refuses it. While “pre-armed”, the NAV ARM flag blinks in a slow rhythm. When it fails to capture, toggle NAV mode off and on again.
• The flight director command bars on the attitude indicator can be hidden via the Disable Flight Director toggle in the EFB Config tab.
• [NEW] Lateral and altitude-capture logic now runs natively inside the Working Title GNS 530/430 flight directors, which fly the mod’s own VOR/LOC/GS signal.
• [CHANGED] the NAV “pre-arm” bullet above (~262): relaxed NAV arming can now be switched off with the new “Disable relaxed NAV arming” Config-tab option.

Cockpit instruments

Work in process: many gauges are getting an overhaul, giving them some physicality and changing their behavior, based on reference videos, manuals, pilot feedback and filling in the gaps with educated guesses.

• Implemented Captain-side altimeter with aneroid simulation, a servo motor that drives needle and drum with realistic latching mechanism, the baro knob is mechanically linked. Based on IDC Encoding Altimeter 28V.
• Equipped Radio Altimeter with a highly reponsive servo.
• Implemented gyro “simulation” for Attitude Indicators and Horizontal Situation Indicators (KI 525A).
• Enabled a bunch of instrument flags that were hidden in the gauges, probably because they’re not properly animated, fixed the behavior of other flags.
• Reimplemented chronometers with clock, stopwatch, block time and an OAT display. I took some artistic license. The mode button switches the clock between local time and UTC, and OAT between C and F. If you find out what else it can do, don’t tell anyone.
• Work started on calibration differences between mirrored instruments (altimeters, HSI, RMI).
• The whiskey compass reacts to windshield heat.

Engines

The goal is: pilot can make engines 'splode.
State: laying the groundwork.

Part of the mod is a turbine “simulation”, aka “TurbSim”.

Let me be perfectly clear about this: while my goal is to present something that is closer to realism than the oversimplified turbine simulation MSFS provides, I don’t know at all what I’m doing here. I vibecoded a basic PT6A simulation based on Wikipedia and publicly available research papers (which are already simplified, not based on actual 1st party data, and cover other PT6A variants), without any insights into the P&W’s “secret sauce”.

I have a bunch of formulas that try to approximate certain parts of the engine, and I feed them with values that I guessed based on what I want the output to look like to hit targets I found in reference videos. I have never seen a hot start, but when the formulas see fuel pooled in the engine before combustion starts, they can make the ITT go red. It’s half physics, half fudge.

Known issues: No hung starts, because of sim limitations. No performance degradation or engine failures implemented yet.

• Calibrated engine fuel flow and N2 mapping for accurate power response.
• Adjusted engine torque-to-N1 mapping to align with POH performance data.
• Added intake deflectors aka inertial separators, enabled doll’s eyes indicators, added torque and ITT penalties for deflectors and inlet heat. Note: it takes about 70% Ng to create the bleed air pressure needed to extend the deflectors.
• Physics-based “simulation” of the PT6A gas path (TurbSim): two-spool dynamics, staged fuel nozzle with characteristic light-off ITT spike, fuel pool that drives hot starts, ignition-delay light-off, T4→T5 gas path with thermocouple lag, block thermal soak-back.
• Per-airframe deterministic jitter spreads engine bias across saved airframes. No two engines are equal.
• Inertial separator needs bleed-air pressure to extend; electrical power is still enough to retract.
• Mitigated the push-from-idle ITT spike by tightening the low-Ng Wf acceleration ceiling.
• A small low-Ng FCU metering-valve servo lag (first-order, ~0.2 s) models the hydromechanical metering valve. No effect on startup (handled by a separate FCU schedule) or cruise/takeoff power.
• Cross-engine-start interlock: starting one engine no longer lets the other engine’s running generator back-feed the starter. The opposite generator briefly drops offline during the crank (its GEN annunciator lights) and re-engages once the starter releases. This is a workaround for a sim electrical-system limitation; per the POH (and physics), a cross-engine start isn’t possible: the running engine’s generator hasn’t got the oomph to drive the opposite starter.

Fuel system

• Migrated to “modern fuel system” (MSFS 2024) architecture.
• Restored the IRL behavior of the wing tanks: the “Engine” position enables the electric pump that feeds fuel from the wing tanks into the main tanks.
• A valve will pause the transfer around 75-80% per target tank.
• The wing tanks “refuel” position does nothing.
• Wing tank pump failure warning lights illuminate when the pumps can’t draw fuel.

Electrical system

The goal was: create the dimming effect that Aerosoft captured, but organically. Instead of scripting the effect based on the starter switch, it should be based on sagging bus voltage.
State: Achieved in the first Milestone release v2024.3.68.

Known issues: I could not figure out how to let two power sources share the load on one bus. The solver always prefers one engine and puts the complete load on it, while the other generator idles. If you have advice, please share it. All batteries are always full at the beginning of a flight (unless you save the state manually in a .flt file, I think.) It also takes a couple of seconds for the first generator to come online after toggling its switch, and during GPU starts the voltmeter can move erratically.

• Rebuilt electrical system using v2.2 for more reasonable battery drain and consumer loads.
• Added opt-in option for realistic battery drain. By default, a magical device keeps the batteries charged.
• Added option to select NiCd or SLA battery with different capacities and behaviors.
• Added invisible GPU cart that can be selected using the DC Source selector on the ground. (Same behavior as before, but with different mechanics to satisfy MSFS2024.)
• Added battery packs to emergency lighting. Armed emergency lights turn on when main power is lost and are powered for at least 15 minutes. (Limitation: the batteries are magically charged by the sim when the flight starts, trickle-charge is currently missing because of… erm… topological challenges.)
• Adjusted power consumption of most components, based on POH values, research, or gut feeling.
• Generator Control Unit “simulation”: GCU faults can trip a generator, requiring an explicit RESET to bring it back online; tripping can fire on undervoltage if enabled in the EFB Config tab.
• Battery temperature thermal model: ambient soak, charge/discharge heating, and cooling.
• BUS TIE switch correctly couples or isolates the two main DC buses symmetrically.
• Interior light dimming reacts to bus load.
• Annunciator brightness now uses a per-lamp aging factor.
• Pitot heat split into two independent circuits. Note: the sim still provides only a single pitot-static system feeding both airspeed indicators. That separation is future work.
• Battery terminal voltage now sags under heavy load. It’s most visible during engine start — the voltmeter dips and the cockpit lighting (including the glareshield annunciators) dims as the starter pulls the bus down. Starter current tapers as the engine spools up. A weak battery cranks noticeably slower and takes longer to stabilize at a lower %Ng.
• Fixed the loadmeter-needle animation that barely moved under idle load; it now displays POH-correct values (the scale differs depending on whether the bus is on battery or on generator, see POH).
• Moved the engine starters to the battery bus to align with the POH electrical topology, and added a back-EMF starter model.

Hydraulic system

The hydraulic system now has a pump that draws power when it actuates. You can hear it work when you use the flaps, brakes, or nosewheel steering. It took a bit of trickery to work around sim limitations (or maybe dev limitations, can’t be sure) and I haven’t figured out how to prime the hydraulics, so every flight starts with an empty system and an invisible mechanic using the hand pump.

• Rebuilt hydraulic system using EX1.
• Added brake isolation valve to preserve some brake functionality on system failure.
• Added emergency hand pump to EFB Systems tab to manually restore hydraulic pressure and to prime the system when state is restored.
• Changed brake and sys pressure gauges to show actual values.
• Pump inrush: the hydraulic pump now pulls a brief power surge when it cuts in, before settling to its running draw — visible on the loadmeter.
• System pressure regulation and pressure-gauge calibration carry small per-variant offsets.

Brakes

The parking brake works in two modes:

• Simplified: just toggle the parking brake by whatever means.
• Realistic: to set the parking brake, pull the handle while toe brakes are fully depressed, then release toe brakes; to release, depress toe brakes fully to unlatch the mechanism. (For convenience, pushing the handle works as well, but that’s not realistic, man!)

Limitation: As I understand it, in the real Twotter, the parking brake is a latch that holds the toe brake cylinders in place. I couldn’t figure out how to prime the hydraulic system when the aircraft is loading, so the flight would start with the parking brake latching unpressurized toe brakes. Unpressurized brakes are bad, mmmkay? So, under the hood, even realistic mode must still rely on the conventional parking brake system. This makes FSEconomy happy, too.

Chocks are deployed by default and can be removed using the EFB app. In some situations, they are removed automatically once the hydraulic brake accumulator is pressurized.

• Adjusted tire friction so she does no longer run away during run-up to 30 PSI torque.
• Rebuilt parking brake logic to better match real-world behavior.
• Added option to switch between simplified and realistic parking brake behavior in the EFB Config tab.
• Aircraft spawns with invisible chocks on. Toggle chocks from the EFB Systems tab.
• Parking brake logic reworked.
• For runway starts, chocks deploy on load and release when brake pressure has built up.

Bleed air & ice protection

The Twotter is certified for FIKI (flight into known icing conditions). It has a beefy de-icing system consisting of leading edge boots on the wings and stabilizers, heated propeller edges and engine ice protection. Because of limitations of the sim, all these systems collapse into one single value that controls the structural icing of the aircraft and its effects on the flight model. I still tried to make each component matter.

• Each boot cycle takes about 16s, and in the last 8s, the blue overhead stab lights will come on.
• A single boot cycle can be triggered using the MANUAL switch position.
• In AUTO mode, boots will be actuated every 3m (SLOW) or every 1m (FAST).
• At the end of each boot cycle, a certain percentage of the ice is shed.
• Activating Prop Heat increases the percentage (yeah, that’s the unrealistic part).
• Activating Intake Anti-Ice increases the percentage (same).
• Extending Intake Deflectors protects the engine from ingesting ice that is shed by Intake Anti-Ice (as part of a future engine damage modelling)

Notes: Bleed Air switches don’t need to be “on” to extend the deflector. The extension mechanism gets its air upstream. At least one switch needs to be “on” to use the de-icing boots or cabin heating (not implemented yet).

Limitations: MANUAL mode does not behave authentically, it just starts a single boot cycle. IRL, in manual mode, the two extra switches (stab left/right, wing inner/outer) SOMEHOW control the boots directly, but I couldn’t find out how, also, it would be complete theater, because of the “one single value” restriction.

Known issue: During the boot cycle, the LOW PNEUMATIC PRESS light can come on temporarily. The boot cycle will complete anyway.

• Rudimentary bleed-air “simulation”: per-engine source PSI, manifold PSI after the cockpit shutoff switches, and demand from downstream consumers (deflector, deice boots).
• Pneumatic de-ice boot system with AUTO and MAN cycle modes. AUTO runs continuous cycles; MAN fires a single cycle and the switch snaps back.
• Windshield heat circuit mapped to separate DC buses (left and right pane independently).
• Icing accretion statistics on the EFB Flight tab.
• Inertial separator deflector deployment now needs bleed-air pressure.
• Windshield heat is now thermostat-controlled: instead of running flat-out whenever the switch is on, it warms the glass and then cycles to hold temperature. The underlying thermal model is minimal — in flight in cool air the heat stays effectively always-on, so it never interferes with the sim’s internal windshield-icing effect (which the mod can’t actually see). The thermostat state and modelled window temperature are shown on the EFB Debug tab.
• CCM applied his signature 400% icing bump, it remains to be tested whether the de-icing can keep up.

Wear & Tear

The aircraft now has an optional Wear & Tear system that ages and damages individual components. This is a vertical slice, covering only two systems:

• Pitot Probe heating elements can burn out if left on for an extended time without airflow. Abuse leads to stress, when stress grows above a threshold, it leads to damage. Stress decays and is ephemeral, damage persists and leads to failures. Damaged components can be repaired or replaced. Test scenario: with the reliability slider not in the “Off” position, in a powered aircraft on the ground with stopped engines (so the Maint tab is available), turn on Pitot Heat and select the Maint tab in the EFB app. You should see the stress and damage bars change.
• Annunciator bulbs age and can burn out. The age affects the brightness. Test scenario: when the aircraft is loaded, all the glareshield annunciators have the same brightness; move the reliability slider from the Off position or click “Re-roll” on the Maint tab, and the brightness of some of the bulbs should change (the effect can be subtle). Each time an annunciator lights up, the filament has a tiny chance to burn out (based on operating time, cycles, current bus voltage; that was some fun math, please tell me if I got it wrong and the bulbs die too often).
• Engines track damage to the hot section. Work in progress.

These systems cover most of the planned wear & tear behaviors: random, physics-based failures of non-essential systems (like annunciator lights), and strictly deterministic stress and damage caused by pilot action, that can lead to (possibly critical) failure.

• The Wear & Tear system is controlled by the Reliability slider on the EFB Config tab: Off, Reliable (nominal wear), Unreliable (4× wear), or Not Airworthy (16× wear).
• A new Maintenance tab in the EFB (available while on the ground with engines shut down) lists every component that has accrued stress or damage. Re-roll randomizes the aircraft state (production year, TTAF, and all component states); Request Service runs maintenance and produces a per-component report.
• A separate Reset button on the Debug tab returns everything to a factory-new state.
• Wear state persists per variant in the WASM work folder.

Bug fixes

• Corrected emergency exit lights logic; they now illuminate correctly in the ARM position upon loss of power.
• Fixed HSI NAV flag logic: flag is now always hidden when GPS drives NAV1.
• Updated ADF circuit to 31 to match standard electrical configurations.
• Restored some flap actuator sounds (see Known Issue).
• Gated cockpit lighting and spoiler triggers to stop spamming events.
• Annunciator lights work again (regression introduced by SU5 release 1.7.21.0).
• Fixed engines quitting during the cinematics on a runway start (both engines), and the related case where one engine would occasionally quit right after the cinematics ended.

Variants

• Tundra Pax and Cargo: committed to the tundra tires feel.
• All Pax variants: added passenger layout with individual seats.
• Amphibian and float variants: increased parasitic drag for the floats.
• Added preliminary PT6A-20 tuning for the DHC-6-100 variants.
• Variants with legacy tuning are no longer included. Stick with v2024.3.68 if you prefer the Aerosoft/Metzger tuning.

Known issues

• Some audio is shit. Aerosoft’s Wwise sound bank limits what I can do. Creating a new sound bank is outside my skillset. Help would be appreciated.
  Example #1: engine startup sounds are inconsistent and don’t adapt to the physics; the sound bank fades between effects based only on prop RPM, it doesn’t care about turbine RPM at all. That’s why an engine start with feathered props sounds so horrible. Also, the interior and exterior sounds are wildly different (just compare the interior starter sounds with the exterior version) and switching between interior and exterior can “restart” audio effects.
  Example #2: Stock flap sounds are not continuous.
• There are four “phantom tanks” visible in the Weight & Balance screen of the EFB. I added them to maintain compatibility with FSEconomy after enabling the modular fuel system. This is a purely cosmetical flaw.
• On the world map, the aircraft thumbnail is missing and a default MSFS2024 thumbnail is showing instead. To fix this, I would have to create and maintain a separate set of “modern” (MSFS2024-style) thumbnails for each variant. That doesn’t seem worth it. On most other menu screens (e.g. when selecting a variant), the sim should use the existing “legacy” thumbnails.
• Generator load sharing: only one starter-generator actually supplies the bus at a time. As far as I can tell, MSFS 2024’s V2 electrical system has no mechanism for two paralleled DC generators to share load. The cockpit loadmeter therefore shows Gen 1 near full load and Gen 2 near zero in normal operation; that’s expected, not a bug. If anyone knows how to get real V2 load sharing working, I’d love to hear about it.
• ADF: the system can’t react to lightning — the sim doesn’t expose the necessary data.
• FLC mode works when the pitot tube is frozen. Somehow, it knows. Sim quirk.
• If you overstress the aircraft or crash into terrain and the sim puts you “back on track”, internal states can desync and lead to failing systems. My recommendation: avoid overstressing the aircraft or crashing into terrain.
• Pilot and Copilot avatars are not visible.
• KA 51 Slaving Accessory is missing.

• Wear & tear, failures, engine damage, including effects; scope as of yet undetermined
• Support for four-blade props (research required)
• Pitot-static system
• Cabin climate
• more DHC-6-100 tuning
• Implement missing TwotterX features