High altitude stall characteristics of jet transports
 

Someome sent this to me recently:


Simulators Still NotAccurate Enough On Stalls
As experts struggle to identify whythe crew of Air France 447 lost control of their A330 over the South AtlanticOcean nearly four years ago, the industry is also still struggling to developthe precision data needed to accurately reproduce a stall in a Level Dsimulator.
The lack of accurate stall data limits entry and recovery practice because thecomputers running the simulators have no idea how the aircraft will actuallyperform.
Safety experts believe better data is needed to properly prepare pilots for aportion of the aircraft’s performance envelope that was once thought easy toavoid.
At a recent conference held at the RoyalAeronautical Society in London, officials from both Airbus and Boeingjoined forces to explain the situation to date as well as where the industrystill needs to go.
Airbus test pilot Terry Lutz believes the day may be coming when pilots willneed to hand over more control to onboard computers when the situation becomestoo chaotic.
This is reminiscent of the blue “level”button in use aboard the four-place Cirrus SR22 piston single thatautomatically brings the aircraft back to a wings-level attitude even if theautopilot is turned off.
Boeing engineer Paul Bolds-Moorhead reiterated the monumental task of developingaccurate lift and stall data in the high-altitude regime, where the stall andunusual-attitude behaviour of transport aircraft is typically never tested.

So, the industry is also still struggling to develop the precision data needed to accurately reproduce a stall in a Level D simulator.

If that is true, meaning fidelity is not assured, then why is high and low altitude stall recovery still a required sequence in the type rating syllabus for various jet transports. I find it hard to comprehend that the certification of the real aircraft does not include high altitude stall recovery flown by the test pilot.

I believe Brian Abraham posted something on this on the AF447 Thread No. 11, just started.

Re, "If that is true, meaning fidelity is not assured, then why is high and low altitude stall recovery still a required sequence in the type rating syllabus for various jet transports. "

The actual stall is not a required sequence for training and checking in any syllabus. Normally where required, (and it is not universally required by all regulators), it is the approach to the stall, where buffet begins and the aircraft's stall warning / stick shaker / stick pusher etc activates. Recovery is expected to begin at that point and Level D simulators up to that point have, I believe, been validated. It is aircraft behaviour in a full stall that does not have data from actual flight testing.

Thanks PJ2 - that explains things. You are quite right; the exercise is to stick shaker.

What prevents the sim programmers from modeling at least part of the stall characteristics of the Airbus off of the AF447 FDR data?

This presentation is 1:40 long, but well worth takin the time to watch. It's a joint presentation from Boeing and Airbus on stall testing of large transport aircraft.

AW&ST had an article on this subject. Apparently a generic sim program that is fairly close to the actual stall characteristics has been developed. We'll see if we get training using the software in the years ahead.

An aspect hinted at in some other threads is the reduction in available engine thrust at high altitude. Thus the SOP "powering through" the stall recovery, i.e. minimum loss of altitude, has a big compromise built in; You linger on the ragged edge of a stall for a LONG while.

Better to drop the nose to a) directly reduce AOA, and b) convert some potential energy to kinetic, to recover faster. (Adding thrust too, of course) The only remaining question is "How much do you drop the nose?"

Stall recovery technique was re written to emphasise reduction of AOA as a first action shortly after AF447 and your manuals should have been updated to reflect this.

I know all ours were for several different types.

If i remember a nose down of -5/-10 deg was quoted in the video.

Approved data. Both Airbus and Boeings are being economic with the truth. They CHOOSE to limit the data within their datapacks. The sim manufacturers are basically stuck with what they are presented with, not what is available within the airframe OEMs flight test data.

If you start the video I posted at about 50:20 it talks about briefed recovery actions when testing. The "LOWER THE NOSE" is firmly emphasised, and the interesting point for me was that your stall margin decreased due to Mach effect as you accelerate. The point made was one must accept trading 2-3000' of altitude for speed in the recovery.

Current sim behaviour is modelled from extrapolations of the data captured during the initial flight tests of the type as well as theoretical data based on the aerodynamic properties of the airframe. It may not be 100% precise, but based on the simulated data presented by the BEA in the report versus the DFDR data from AF447, it's very close.

I think the article the OP refers to takes a few quotes and derives conclusions that are inaccurate - for example:

The computers actually have a very good idea of how the aircraft will perform (as I said above) - it's just that the data used is based on extrapolation and theory rather than physical testing (and yet it's very close). I was fortunate enough to sit in on experiments in a Level D sim and the behaviour seemed very convincing.

The B737 FCTM states that for holding above 25,000 ft with inop FMC, fly at Vref40 plus 100 knots. In the 737-300 that means around 230 knots plus or minus a bit.

Therefore that figure is useful as an aiming point when recovering from a high altitude stall where you lower the nose to zero body angle to unstall the wings and maintain that body angle until reaching 230 knots IAS. It is then safe to level out. It means deliberately losing around 3000 ft of altitude because that is what it takes to accelerate to 230 IAS allowing for low thrust levels at high altitude.

Yes, the video mentioned -5/-10 nose-down, but also emphasized that recovery is (or should be) started as soon as the airplane is considered stalled, i.e. at g-break or deterrent buffet.

How much to drop the nose?
 

Maybe the answer should be found on the "forgetted and placed ..x screens down" AOA indicator?

As SLF I thought there were parallels in the following incident with 447 ie time taken to recognise the aircraft was stalled and the height lost.

AEROMEXICO DC-10-30, NOVEMBER 11, 1979

The flight recorder data for this incident showed a constant rate of climb with continually decreasing airspeed before buffet onset and sustained aircraft stall.

According to the crew, while climbing through 27,500 ft, they felt a vibration which, within seconds, increased in intensity. The crew suspected an abnormal vibration in engine No. 3 and elected to reduce its power and then to shut it down. The crew also stated that, upon reducing power on engine No. 3, the aircraft assumed a pitch down attitude, the AP became disengaged, and the aircraft rolled to the right and then to the left and started to lose altitude.

The Digital Flight Data Recorder (DFDR) revealed that, after the No. 3 engine power was reduced, the aircraft decelerated into speeds that were below the stall buffet speed and the design flight envelope. Shortly thereafter, the nose dropped and the aircraft entered into a stall while at 29,800 ft and an IAS of 226 kns.

The calculated stall speed for the flight a t the time of the occurrence was 222 kns. The calculated buffet onset speed was about 241 kns. The DFDR showed a constant rate of climb until the stall and loss of altitude occurred. It also showed that the airplane noseup elevator was held between 9° and 18.2° throughout most of the recovery maneuver until the elevator was gradually relaxed with recovery from the stall starting at about 24,500 ft.
The DFDR readout showed the recovery started at 23,900 ft. At that time, the airspeed increased to a value above the calculated stall speed. The vertical acceleration reached a maximum of 1.68 g's during the recovery process which ended at an altitude of 18,900 ft, and the crew regained full control of the aircraft about 18,000 ft. According to the crew, when aircraft control was lost, the first officer declared an emergency. During that period, the DFDR showed that the aircraft was responding in a normal manner to crew control inputs.

It was found that about 4 ft of each outboard elevator tip, including the corresponding counterweights and the aircrafts tail area lower access door were missing. Numerous cabin ceiling panels, light fixtures, and an oxygen mask had been dislodged; however, most of these had been reinstalled by the cabin crew.

21:40:56 to 21:41:16

Aircraft IAS decreased from 247 to 226 kns while in a steady climb profile from 29,510 to 29,834 ft.

Aircraft pitch attitude increased from 8° to 11° noseup. Roll attitude went from wings level to 14° right wing down.

The horizontal stabilizer was deflected from 4.2° to 6.0° noseup. The aircraft heading changed from 264° to 271° and the aircraft entered into a buffet onset speed and later into a prolonged stall.

21:41:16 to 21:41:26

IAS decreased from 226 to 208 and then to 197 kns as the aircraft descended through 29,600 f t while still in a stalled condition. Aircraft pitch attitude increased from 11.0° to 17.4° noseup. The spoilers were deployed and stayed deployed for 75 seconds. The left inboard elevator sensor indicated that the elevators started an excursion from l° up to 12° up and then to 10° up.

The horizontal stabilizer deflected from 6.0° to 6.6° noseup. Although the aircraft was in a stalled condition, the elevators were commanding noseup, and the horizontal stabilizer was trimming for the noseup command. The aircraft heading changed from 271° to 283° and then to 272.7° while the No. 2 engine N rpm decreased to about 90 percent and then began to fluctuate at 100+-5 percent which continued for about 45 seconds.

21:41:26 to 21:41:36

Aircraft IAS decreased to 178 kns as it descended through 28,900 ft in a stall. Aircraft pitch attitude decreased from 17.3° noseup to 14.8° noseup.

The lower rudder sensor indicated that the rudders were deflected from 1.4° right to 11° left, which was beyond the 5° authority of the yaw damper.

Vertical acceleration remained about .9-g loads. The elevators continued in an excursion from 10° up to 8° up and then to 19° up.

The horizontal stabilizer deflected from 6.69° noseup to 8.33° noseup.

The aircraft heading changed from 272' to 274' and then to 272'. The rate of descent was reduced from about 4,200 to about 600 ft per minute.

21:41:36 to 21:41:56

The aircraft IAS decreased from 178 to 175 kns and then increased to 217 kns as the aircraft continued to descend through 25,600 ft at about 10,156 ft per minute.

The aircraft pitch attitude decreased from 14.8 noseup to 10.9° nosedown and then to about 6.6° nosedown.

The roll attitude continued an excursion from 3° left wing low to 23.5° left wing low to 25° right wing low to 3° right wing low to 13° right wing low and then to 5° left wing low. The rudder deflected from 12° left to 3° left.

Vertical acceleration changed from .9 to 0.65 to 1.0 g.

The elevators oscillated from 17° up to 9° up and then to 16° up.

The horizontal stabilizer deflected from 8.33° noseup to 9.46° noseup and then to 6.48° noseup.

The aircraft heading changed from 272° to 264° and then to 278°

21:41:56 to 21:42:06

The aircraft IAS continued to increase from 217 to 248 kns as the vertical speed continued to increase to 15,000 ft per minute rate of descent at 23,300 ft. The aircraft vertical acceleration changed from 1.0 to 1.4 g.
o The elevators deflected from 13.7° up to 8.4° up as the stabilizer deflected from 6.48° noseup to 9.56° noseup. The heading changed from 278° to 276°.

21:42:06 to 21:42:16

The aircraft IAS increased to 267.5 kns as the vertical speed slowed to about 11,988 ft per minute while descending through 21,600 ft.

The aircraft pitch attitude started an excursion between 5.2° nosedown to 5.7° noseup. The roll attitude went from wings level to about 5.7° left wing down.

The vertical acceleration oscillated between 1.4 to 1.1 to 1.68 g (the highest g load experienced during the occurrence). The elevators changed from 8.4° noseup to near neutral as the horizontal stabilizer increased to 9.87° noseup. The aircraft heading remained nearly constant at about 276°.

21:42:16 to 21:44:08

The aircraft IAS decreased as recovery became evident through a decreasing rate of descent and coordinated maneuvers which started about 21,600 ft and ended in a level controlled flight about 18,900 ft. The sequence of events was appropriate for a stall recovery in contrast with the sequence of events preceding 21:42:16 during which it appeared that the aircraft control inputs were correcting in the wrong direction for a stall recovery.

The theoretical stall speed of the aircraft for its climb weight was determined to be 203 kn and the buffet onset speed according to the Aircraft Flight Manual was approximately 234 kn. According to the DFDR, the aircraft was operated below 234 kn for over 40 seconds while climbing between 26,000 ft and 32,000 ft. During half of this period, the airspeed was below 203 kn. That the aircraft pitch attitude decreased from over 14° noseup to over 10° nosedown while nearly full noseup elevator deflection was held clearly indicates that the aircraft was in a fully stalled condition. Although the crew failed to recognize the approach and entry to the stall, they did, after approximately 1 minute, recognize the aircraft's stalled condition and responded with proper control to recover. A full minute for stall recognition is excessive. However, the DC-10's stall warning system consists only of a stickshaker, the operation of which might be misinterpreted by an inattentive or distracted flightcrew, particularly when the aircraft is controlled by the autopilot rather than a pilot. Although the flightcrew on this incident was not attentive to the aircraft's condition, a more explicit stall warning device might have alerted them sooner to the aircraft's true condition during its approach to the stall. We note that some transport aircraft, in addition to a stickshaker, have both visual and aural stall warning devices. We believe that either of the latter would have more quickly resolved the flightcrew's stall recognition problem and might have prevented damage to the aircraft. Consequently, since stall problems can be encountered by a legitimately distracted flightcrew, we believe that the stall warning system in the DC-10 should be improved to include either a visual or aural warning device, or both.

The flighcrew misinterpreted the stall buffet or the stall warning stickshaker or a combination of both as a No. 3 engine vibration.

Stall recovery procedures were implemented approximately 1 minute after stall entry and a successful recovery was effected.

The total altitude loss from stall to complete recovery was approximately 11,000 ft. The aircraft did not exceed VmoIMmo and neither aerodynamic load limits or acceleration limits were exceeded.

The stall buffet which was encountered as the aircraft approached and entered the stall produced a dynamic load on the elevator balance weights which resulted in structural overload and failure of the outboard elevator tips.

The preface to the Aeromexico DC-10 case is that the aircraft was climbing in VS mode and ran out of thrust margin i.e. IAS deteriorated to the point of stall.

Not unlike the Pinnacle RJ, 14 Oct. 2004, two cowboys on ferry flight made it to FL410 before aero stall/engine stall/flameout. :ouch:

How much to drop the nose?

However much is required depending on

(a) what the present angle of attack may be

(b) what the pitching characteristic will permit .. ie one may need to revert to innovative practice .. pitch and power oscillations, roll, whatever

Modest pitch decreases such as 5-10 degrees nose down may be suitable for an immediate response in some aircraft but will be woefully inadequate if the angle of attack is 40-50 degrees or so.

It seems on the B744 with MCT set that a pitch attitude of not above zero provides a good rate of acceleration and a reasonable height loss for recovery in the cruise altitude regime. Any more than -1 would be ill advised IMHO, it's just not necessary. However in the cruise altitude area if there is any problem, stuff the nose down and unstall the wing. That is the primary issue, once the wing is flying again then sort out the rest.

As John says, it depends. It is reckoned AF447 needed to lower the nose about 40 degrees to recover, and it also depends on how 'stalled' you are. Bear in mind when we talk about 'stalling' in airline ops we normally mean the stall warning, some way off being actually 'stalled', so the need for violent manoeuvre is reduced somewhat.