I’m posting this in the “airlines” section in the hope you folks who fly the big stuff might have ideas…
I’m writing a fictional piece in which a T-tailed airliner (CRJ-200) is testing new flight software and gets caught in a DEEP STALL. As most of you probably know, this is where disturbed air coming off the wings at a high AoA blankets the tail (on a T-tail a/c), rendering the elevator useless. It has caused some notable crashes and is considered by most pilots to be “inescapable.”
I have a 1965 NASA Langley simulator study that talks about “low pitch damping and negative pitch damping” being “favorable to recovery,” and an aeronautical engineer mentioned the possibility of rolling inverted to escape it, though that takes you into a yaw area where you could end up in a flat spin… then you’re even worse off.
Are there any “sensible” ideas out there for breaking a deep stall in an airliner (other than of course detecting it while you still have enough elevator to pitch down)?
In ought to be possible to get out of a deep stall by altering the CofG signifcantly forward. How to accomplish this I have no idea. But hey, you’re the writer you’ll think of something .
How about shifting fuel to a more forward CG (assuming you had time/altitude)? I know the CRJ has 5 tanks: 2 mains, one center, and 2 “collector” tanks. Assuming the the mains are wingtanks, it seems like a center tank would be a little more forward, given the wing sweep.
I have no idea about the collectors though.
Anyone know for sure?
It would be faster to have the passengers run forward. On the other hand they are so panicked they probably wouldn’t hear you anyway. At 10,000 Feet per minute whatever you do better be quick.
P.S.
If the software was that critical the test would be done by the factory in a test airplane that has the drogue chute in the tail which is there just for this reason.
Answering the previous three Q’s (and still laughing over the crocodile one)… Passengers – There are none; it’s a test flight.
**Lab test Vs real **-- Yes, but it wouldn’t make for good fiction.
No drogue chute, as test was supposed to stop short of an actual stall (again, it’s fiction guys). [Note: Canadair CL-600-1A11 (T-tail) w/drogue chute got in a deep stall and subsequent spin back in 1980. The chute failed to detach, forcing the test crew to bail out. One died when his own chute failed to open]
**Engine thrust **-- Engines are mounted aft, on fuselage, so not much turning moment there.
I realize this is difficult, but at least we’re not aboard Hawker Siddeley Trident G-ARPY, BAC One-Eleven 200AB, Canadair CL-600-1A11, or Tupolev RA-85185 – all of which pancaked into the ground after getting into deep stalls. The Brits often refer to it as a “stable stall” or "super stall, though neither conveys the terror evoked by “deep” (as in deep doo-doo).
Well, aside from the normal gear and flaps options, I’d be likely to pop out a thrust reverser or two. You’re probably going to end up rolling on your back and entering a spin, but at least your in a nose down position with s possibility of recovering. Either than or you make a spectacular exit!
Since you nixt the drogue chute… your left with excess thrust to fly out of it… oh wait it is a CRJ-200. Never mind they’re F*&%^d.
Chop power on one engine, let the adverse yaw roll her over into a dive, attempt to recover.
Challengers and their stretched CRJ-200 brother are CG picky.
Bombardier crashed one and lost two of their test pilots doing an aft limit CG flight test on new software. The aft shift of fuel during the takeoff acceleration was enough to cause a loss of control, and stall on takeoff.
The only other option would be a sliding ballast on a rail moved with pneumatic or electric actuator. The Air force and NASA have used this setup to test extreme CG shifts on larger aircraft.
In a deep stall passengers would not be able to overcome the cabin incline (45+ deg) to move forward.
Similarly the engine thrustline would be canted and power changes would probably have little or no effect on horizontal attitude, although judicious asymmetric thrust might enable a wing to lift and gently roll out of the stall. I don’t know that anyone has done this successfully. If the compressors have stalled (likely due to no intake airflow) the this won’t work either.
F-16 pilots are taught to rock the elevator but this isn’t relevant since F-16 “deep” stalls are software-induced in the first place.
If your scenario is a test flight then you could have extra cabin tanks installed and quickly! pump fuel into the most forward one. A bit deus ex machina but I think you’re allowed some license .
Okay, lots of ideas here… Thanks.
I was happy to see someone besides me considered thrust-reversers. Granted, accidental TR deployment didn’t work out well for Luada Flight 004, but unlike that tragedy we’re in a deep stall here, with virtually no forward airspeed and maybe 4,000fpm vertical velocity.
**IF **(huge “if”) you could deploy the thrust-reversers, **AND **you didn’t already have a comprssor stall at this high AoA (another big “if”), **THEN **would the effect be like standing on a speeding cat’s tail, thereby jerking his wet little nose earthward? Or would you just tear both engines (or the tail) to pieces and crash anyway?
Good point (duh). Didn’t have thrust reversers on my Archer, so am obviously in over my head here.
But purely as an academic question, if forward momentum is virtually nil (we’re “pancaking” here ), isn’t the aft CG problem already at its maximum? Not trying to be a smart*ss–just asking…
The point isn’t to use reverse thrust, it is to use ONE reverser to create a tremendous amount of drag on one side of the aircraft. A thrust reverser deployment in flight has a very good chance of rolling the airplane over on it’s back, kind of like doing a really good spin entry in a Cessna. Spin recovery anyone? It may not be a great option, but it’s better than just sitting there as the ground rushes up.
Mark is right, deploying thrust reversers on aft mounted engines creates a nose up moment. Conversely increasing thrust will help the nose down.
Somebody needs to check to see if the T/Rs on the RJs can be operated in the air. They may be locked out by the squat switches being in the air mode.
Good thoughts – thanks guys. It sounds like TR deployment (one or both) is too risky.
This may be a good time to mention the 4th–and final–conclusion in that 1965 NASA report I cited in my original post (“A simulator study of T-tail aircraft in deep stall conditions” – http://ntrs.nasa.gov/search.jsp?R=29316&id=4&as=false&or=false&qs=Ns%3DArchiveName%257c0%26N%3D4294962896%2B42 ): “4. Lateral characteristics must not be ignored in stall recovery performance estimates, because it appears that for some configurations control of lateral instability may be an overriding risk.”
I think they mean the risk of entering a spin or going inverted, right?
Keeping in mind it was a simulator study, they recommended “low pitch damping and negative pitch damping” as the best chance for escape.
Translation please?
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