Question for Real World Pilots: Flaps at High Altitute

[Cjr60611 31]

Set-up: Flying the JRollon CRJ-200 in X-Plane 10.45 from KORD to KBUF with SkyMaxx.  Weather was overcast with cloud ceiling at 13000ft but no heavy winds or turbulence.

 

Issue: Cruising at 31,000ft I began my decent into KBUF about 90 miles out.  At about 28,000ft I inadvertently pressed the wrong button on my joystick - TWICE.  This caused the flaps to engage at about 420kt.  I immediately realized my mistake and attempted to retract the flaps but it was too late and the plane began an uncontrolled drive that I was completely unable to recover from crashing to the ground within 45 seconds!  

 

Question: If this happened in real life would the aircraft really respond in that way?  Of course, I wouldn't be surprised if the flaps completely broke off at that speed but would that cause an unrecoverable dive/free fall? Other than my curiousness about all things aviation, I am also wondering if X-Plane exaggerated the situation?  

 

I once ran into extremely heavy turbulence flying over Florida in XP.  I was indeed able to recover  but I couldn't help but wonder was XP exaggerating the turbulent affect on the aircraft?  Only questioned because I had never run into that type turbulence in years with FSX and P3D.  Likewise, I'd never lost control of a plan like that in FSX or P3D either.  Just wondering if I need to somehow adjust my realism settings differently or, are these events, and outcomes, completed plausible? 

 

Any insight would be great.

 

BTW: *Yes, I made an absolute "Bone Head" mistake!  As an armchair sim pilot, nobody was hurt other than my fragile ego - so you can answer sans the Bone Head Back Slaps!

[KevinAu 441]

Typically the flap limitation is 20000' maximum altitude for deployment. So what happened in your sim is probably as good as anybody's guess on what would happen if extended there since they were probably never even tested in that regime. Having said that, I do know of an incident where somebody grabbed the flap handle instead of the spoiler handle on an airbus in a similar event and they survived with not much more than a rumble.

[Cjr60611 31]

Typically the flap limitation is 20000' maximum altitude for deployment. So what happened in your sim is probably as good as anybody's guess on what would happen if extended there since they were probably never even tested in that regime. Having said that, I do know of an incident where somebody grabbed the flap handle instead of the spoiler handle on an airbus in a similar event and they survived with not much more than a rumble.

 

Thanks for the insight!  Good to know...

[michal 44]

the plane began an uncontrolled drive that I was completely unable to recover from crashing to the ground within 45 seconds!  

Most likely you went beyond software's design envelope - what you got was what is commonly called an 'undefined' result.

[steve310002 1,198]

Excellent question and something I too have often wondered as to what the result would be at high altitude and or high speed. In Prepar3d, nothing much happens other than a 'your flaps have been damage due to excess speed' warning. I have often wondered if the strength of the flaps would withhold the resulting force of drag if extended at high speed accidentally.

[vololiberista 205]

In the VC10 there is a flaps isolate switch that locks them up until you need them for approach. All a/c have flaps limitations speeds and this also includes Mach. But in the sim it's down to whether the developer actually modelled flaps limitation and whether it was even tested. So don't always expect the a/c in the sim to be an accurate representation. It's rare actually!!

[ErichB 3,642]

Desktop sims generally replicate plausible flight under 'normal' operating conditions.  I wouldn't read anything into sim behaviour in abnornal scenarios.  

[FormerSF3 15]

Was it 420 indicated or true airspeed?

 

Some manufactures have the flaps locked out above certain speed while others do not.  Both have resulted in accidents.  One they told us about was a MD11 Flap handle was bumped in cruise.  It nearly rendered the aircraft uncontrollable.

 

Other manufactures have a flap blow back device in the hydraulic system to prevent damage.

 

If you talk to an old 727 pilot, they used to pull the slat circuit breaker and deploy the flaps to 1-2 degrees in cruise.  However that practice was outlawed.

 

See TWA flight 841.

[qqwertz 781]

Interesting question.

 

Just theorizing, but I would expect that due to thinner air, flaps could withstand higher true air speed at high altitudes (though probably not 420 Kts). At the same time, extending the flaps would likely result in a turbulent air flow, so you would lose lift and have increased drag. I guess you would lose control of the plane while the flaps are extending, before the flaps actually break.

 

Peter  

[Cjr60611 31]

 

 

Was it 420 indicated or true airspeed?

 

Good question, I was so busy trying not to lose my flight that took almost an hour to plan that I only glanced at the airspeed for a moment.  Pretty sure it was 420kt TAS.

[vololiberista 205]

Early Caravelles used to extend the flaps to slow themselves down at high altitude. Causing loss of the whole wing!!!

Flaps are simply not designed to be in such a fast air flow all be it in thinner air.

[Cjr60611 31]

Thanks for the insight all, interesting stuff!  Based on your responses, I'm chalking the massive fail up to pilot AND simulator error.  Gotta change the button configuration on my control unit, I was actually attempting to adjust the Trim.

[fnav77 56]

On the Boeing 777, above 20000 feet, if you try to extend the flaps, nothing will happen, the flaps system is inhibited to prevent any unwanted deployment.

[woodhick 504]

Man-Oh-man, some of those (RW) scenarios are scary!!  Glad all sims come equipped with that pause button

[skelsey 765]

This is, as so often, quite type-specific.

 

The general issues are going to be related to the change in airflow and/or the Centre of Pressure that results when the flaps are extended. The latter, in particular, may result in 'undesirable' pitching moments, which may be either up or down depending on the original positions of the CofG and CofP, plus the effect of the altered airflow on the other aerodynamic surfaces (i.e. tailplane) and the effect of the additional drag (which will depend on whether or not the flaps are physically located above or below the CofG). Not all of these effects are going to be equal in size and the response of any particular aircraft will depend heavily upon its specific design, so it's not really possible to talk generically.

 

There are, as already mentioned, limitations applied to the flaps with regard to IAS and Mach (the latter of which is going to be more significant at high altitudes) and again this could be due to higher airspeeds either overstressing the flaps or Mach effects (which may not in itself overstress the flaps but may result in handling problems, again more likely to be the issue at high altitude).

 

TWA841 is an example of an uncommanded (or, as was argued by the NTSB, very much commanded!) slat extension in the cruise on a B727 which lead to some rather 'exciting' manoeuvres (though it should be noted that it was asymmetric extension that was the main cause of the departure from controlled flight, whether commanded or otherwise).

 

Of course, there may well be aeroplanes (I'm thinking mainly those of the military variety) which could very well make use of some flap at high altitude for various manoeuvres.

 

As an aside -- most (all?) Boeing FCOMs have a note about flap extension not being permitted above 20,000ft. Some might (and do) argue the wording implies that the limitation is for the action of extension, not having flaps extended per se. What it most likely means is that during certification the highest the test pilots flew with flap extended was 20,000ft and therefore flight with flap extended above this altitude has not been tested -- it does not necessarily mean that the wings will fall off and the aircraft will instantly go out of control if you fly above 20,000ft with the flaps out, just that it has not been tested and therefore nobody really knows what will happen -- in effect, if you do so you are becoming a test pilot.

 

In any event, selection errors aside it is unlikely to pose an issue in normal operations except landing at very high altitude airports where 20,000ft may only be 10,000ft (or less) above ground level and, particularly coupled with the resulting high TAS and need to get the aeroplane slowed down, it may be tempting to get the flap out too early!