All that is wrong with the MSFS Flight Model (Inertia, Stalling, Pitch Authority, Trim & Sensitivity)

Yes. Up to now I have done flight model mods for 14 Asobo MSFS 2020 airplanes and 9 FSX conversions. I wrote a wishlist article about my “cookbook recipe”, see Workaround for missing propeller drag, please! - Self-Service / Wishlist - Microsoft Flight Simulator Forums

My A320 mod is on A320neo flight model mod • Microsoft Flight Simulator
My Salty 747 is on 747-8i Salty mod + flight model mod • Microsoft Flight Simulator
And A380 is on Airbus A380 generic • Microsoft Flight Simulator

I use a value of 0.05 for the *_stability parameter. This value allows using the “legacy” flight model. Note: I think “legacy” flight model shall be banned.

I started with MSFS 2020. But I have heard that this was the use in FSX. Asobo kept the parameter, but changed the interpretation.

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Awesome work I’ll check them out!

You -can- get behind the power curve on jets … ie your downward inertia can’t be fixed instantly when you jam on the throttle… it takes time for things to spool up.

But that’s not what you were talking about… it’s inadequate lift generated ( or modeled ) in the flare, and the model just plows the plane into the ground.

CFIT is not a good fit here, because you saw the terrain and were flaring to avoid it… however , even though you “thought” you had enough control to flare, the plane didn’t respond to your control input… so… very technically … this would be a bad landing … terrain was not a factor.

Back side of the drag curve does not have anything to do with being unable to arrest the descent during flare. Every approach is flown on the back side of the curve, everything below Vmd (green dot) is. This has nothing to do with the ability to flare, there is still energy left to flare as long as you are flying above stall speed. In other words, flying on the backside of the drag curve with insufficient thrust set the airspeed will decrease, it doesn’t say anything about the energy state of an aircraft.

Similarly downward inertia has nothing to do with being on the back side of the drag curve, you don’t correct downward inertia by applying more thrust, you would lift the nose and add throttle simultaneously if you want to maintain airspeed obviously. During flare you wouldn’t apply thrust.

Problem in the video is that he is flying at a too high angle of attack, if you look at the AOA indicator the aircraft is flying on the critical AOA, no energy left to lift the nose and flare which has nothing to do with the flight model itself, although the stall warning should have activated.

The definition Controlled Flight into Terrain (CFIT) is not a good fit here because the aircraft is not under control. If it would be under control, CFIT does not necessarily mean flying into a mountain, “terrain” in this respect means flying into the ground or water (or runway for that matter) whilst under control.

https://www.skybrary.aero/index.php/Controlled_Flight_Into_Terrain_(CFIT)

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That would be classified as a “loss of control” incident or accident in that case.

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You must be talking about something different or there’s a different definition.
Except for the Concorde I’m not aware of any airliner which flies the approach on the back side of the drag or power curve.

The back side of the drag/power curve is the area of reversed command.
The slower you fly, the higher the required thrust/power.

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Everything behind green dot is the region of reverse command, the Vmd does shift to the left and up when selecting flaps but still most aircraft are flying in the region of reverse command during approach and flare I believe…

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So when you fly the approach stable at 130kts and you reduce the power to maintain 120kts, you need more power to maintain 120kts than 130kts?

On most turboprops, yes, on jets the drag curve is much more flat, so it won’t be noticeable I guess. The only thing I could find is that the flap / slat maneuvering speeds on the A320 (B737 as well as far as I remember), although primarily being based on stall margins, approximate the best climb gradient speed which means they approximate Vmd in that configuration. Everything below those speeds should be region of reverse command.

Edit: thrust is not the same for the same N1 with airspeed, it is considered as more or less a straight line on a jet engine although this is more correct for a turbojet compared to a turbofan. The thrust decreases initially with increasing airspeed due to momentum drag until recovered by the ram rise and then rises slightly with increase of airspeed.

So maybe you won’t need an increase in N1 after all when flying at a lower speed?

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I don’t think that it would be easy to certify a jet where you need more thrust to maintain a lower approach speed.
Again, the Concorde is the only one I know.

I’ve never flown a jet where this is the case. It’s always lower speed = less thrust/N1/EPR.

I don’t think that a range of ~10kts noticeable affects the thrust/N1 ratio.
You definitely need a lower N1 at lower speeds.
It’s too long ago that I flew the Dash7 and 8 to remember exactly, but AFAIR the difference to jet handling concerning speed/power was negligible, especially on the Dash8 when flying the approach at 900RPM.
I even used to select flaps 5 before extending the gear to fly the Dash8 more jet like :sweat_smile:

Its hard to know where the Vmd shifts to exactly when selecting flaps, the only information I can find is the Slat / Flap maneuvering speeds approximating the best gradient climb speed which means those speeds approximate the Vmd in those configurations. But it is not an exact science, those speeds are foremost based on sufficient maneuvering margin above stall speed + 15 degree AoB overshoot which has nothing to do with climb gradients or region of reverse command so its a guesstimate… Neither does it has such a big influence on a jet. On anything other than swept wings the drag curve is much steeper and so is the speed instability and thrust / power required.

On the ATR this is noticeable I believe, the aircraft slows fairly quickly to 180 kts when decelerating on a 3 degree glide path and then dwells there for some time after which the deceleration starts to pick-up again…

Anyway being on the back of the power or drag curve (which is also an eternal discussion) has nothing to do with our friend flying a B748 uncontrolled into the ground.

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The classification of “flying at a too high angle of attack” can be “loss of control” or CFIT - in my humble opinion.
The Sunday 22 June 2003 CRJ-100ER accident was classified CFIT. Maybe the pilots made the “flying at a too high angle of attack” mistake in real life.
The report summary says: “As they further descended below the glideslope (25 seconds before impact), the Ground Proximity Warning System (GPWS) began to sound. Engine power was added, but it was too late. Flight 5672, with the gear down, touched down in a field and collided with several obstacles”. See ASN Aircraft accident Canadair CL-600-2B19 Regional Jet CRJ-100ER F-GRJS Brest-Guipavas Airport (BES)

I didn’t read the full report, but I don’t see and cues they were stalling? They performed an unestablished approach, then tried to alter their flight path (including adding thrust) too late and impacted terrain. That would indeed be a Controlled Flight into Terrain accident. Whenever stalled it would be classified as a loss of control.

CFIT:

“Controlled Flight into Terrain (CFIT) occurs when an airworthy aircraft under the complete control of the pilot is inadvertently flown into terrain, water, or an obstacle. The pilots are generally unaware of the danger until it is too late.”

Source: Controlled Flight Into Terrain (CFIT) - SKYbrary Aviation Safety

Loss of control:

“Loss of control usually occurs because the aircraft enters a flight regime which is outside its normal envelope, usually, but not always at a high rate, thereby introducing an element of surprise for the flight crew involved.”

Source: Loss of Control - SKYbrary Aviation Safety

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In another report summary I read “The ‘Glide Slope’ and ‘Sink Rate’ warnings continued for the rest of
the approach. … The co-pilot then reportedly increased power and attempted to pull back on the control column, which ‘felt as if it was blocked.’ A few seconds later, the aircraft impacted the ground”
See HINDSIGHT (Page 1) (skybrary.aero)

The report says about cockpit communication:
"At 21 h 50 min 58 s, the GPWS callout “Five hundred” is heard, followed by the “Glide slope” warning one second later.
At 21 h 51 min 02 s and 21 h 51 min 04 s, two GPWS “Sink rate” alarms are heard, the second occurring at the same time as autopilot disengagement.
Between 21 h 51 min 11 sec. and 21 h 51 min 14 s, four GPWS “Glide slope” warnings are heard. During this period, the Co-pilot on two occasions said “come right”.
At 21 h 51 min 15 s, the GPWS callout “One hundred” is heard. At 21 h 51 min 16 s, the Co-pilot said “I’ve nothing in front”, then the Captain says “Go-around”.
At 21 h 51 min 21 s, the GPWS “Pull up” warning is heard.
The first noise of impact is heard at 21 h 51 min 22 s. "

As cockpit situation we have a sink rate alarm that is ignored. We have a co-pilot that says stick ‘felt as if it was blocked.’ As far as I know - no technical defect was found on elevator, but the airplane was destroyed by fire. The puzzle pieces do not fit together. But because there was only 1 fatality, the pilot flying, there was only a brief investigation.

The controls were blocked because the captain had both his hands on the controls, there was nothing wrong with the aircraft, the captain did not respond to the GPWS alerts, did not alter flight path, the first officer tried to alter flight path but did not communicate clearly by calling “go-around” for example, hit the TOGA button, he tried to pull back on controls which, for him felt as if they were blocked because the captain was still controlling the aircraft, that is how I read it. So indeed no technical malfunction, they flew into the ground controlled, hence: Controlled Flight into Terrain (CFIT).

Thank you for this information. I did not know about this “unsafe” feature. In air traffic control we have “hand over” and we have “take over” in our systems. A “take over” is a “I take control” command. This command is logged, but this command does not need agreement from the other station control is taken from.
The “both his hands on the controls” is “hand over” in my world. As engineer I prefer “take over”.

As I understand it there was a breakdown in communication and possibly a pilot incapacitation? The FO should have called “GO-AROUND” twice, if still no response he should called “MY CONTROLS” and taken over the controls, instead he hit the TOGA button and at some point tried to pull back on the controls without clearly announcing the control take-over. At least that is how I interpret that piece of text. Not responding to a GPWS alert in that kind of weather is already a no-no, the captain should have announced that he is correcting back onto the glideslope or below stabilization altitude immediately executed a go-around. They didn’t take full advantage of the automation onboard, approach mode coupled to the flight director or use of autopilot, like always there is a chain of events.

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Yes, my experience, too. Today four or five things have to go wrong before an accident can happen. And it is very good that a single mistake - in nearly all cases - can not cause a disaster.

Just a couple of points regarding the drag curve in swept winged jets and thrust that may help those who aren’t familiar-

I have 425 carrier landings, and yes indeed, it takes more thrust to fly slower. Narrow body airliners and biz jets nominally have fairly low sweep angles these days (some notable exceptions obviously, don’t waste my time posting the 747, Citation X and Gulfstream etc exceptions) and are flying Vref in the 1.3 Vso range. Tactical jets coming aboard a ship are flying Vrefs slower than that and due to flap/slat drag and stores, are behind the power curve. The point is, don’t forget what the drag curve looks like in the approach configuration with leading edge slats and the drag of multiple slotted flaps deployed. It takes a ton of power to fly in that regime. Slow one of the airliners down to Vref +5 knots with full flaps and see if it takes more power to stay there.

N1 is probably a bad thrust reference. Fuel flow is likely better, since rpm in a turbojet or turbofan engine isn’t remotely linear with respect to thrust output.

I’m sure you know this, but a lot of folks don’t.

There is indeed an issue with trim, flat plate drag equivalent of windmilling propellers, and control deflection (or lack thereof) with both stick input and trim. I noticed it immediately in the T45C, wondering if I am missing some sort of artificially stability setting that is meant to dumb down the game. The T45C can’t be stalled, or forced into buffet even in tactical maneuvering with full aft stick in my testing.

You’ve highlighted a host of issues that are easily duplicated, meaning that while the “game” is fun to prattle around in as long as you are in the middle of the envelope and just want to enjoy the amazing scenery or fly an approach here or there, there is still a lot of work left on the FM, if ASOBO is interested in getting the flight model right.

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