Are transport category aircraft stalled at high altitudes in certification testing?

[hawk37]

In a discussion with a colleague we realized we had no idea whether manufacturers actually conduct a stall in an aircraft at or near it's certified ceiling. FAR 25 here

eCFR ? Code of Federal Regulations (http://www.ecfr.gov/cgi-bin/text-idx?SID=9de476b2c3adb8178397839c72f0be8d&mc=true&node=pt14.1.25&rgn=div5#se14.1.25_1103)

does not seem to stipulate any altitudes for the stalls to be demonstrated.

A further question came up as to whether compressibility could factor into an aircraft's stall behaviour (I know the TAS will still be slow compared to cruise, however the higher angles of attack near or at a stall can still produce some compressibility effects). Further, perhaps the thin air at such a slow speed, plus any airframe effects could cause a jet engine to flame out.

Anyone in the know?

Hawk

[Gysbreght]

On the first question:

FAA Advisory Circular 25-7C Flight Test Guide For Certification Of Transport Category Airplanes states:

(g) In accordance with § 25.21(c), stalls must be demonstrated up to the maximum approved operating altitude to determine if there are any adverse compressibility effects on stall characteristics. These tests should be flown with gear and flaps up at the most adverse c.g. Power or thrust may be set, as required, to maintain approximately level flight and a 1 knot/second deceleration. A slight descent rate is permissible as long as the stall occurs at approximately the maximum approved altitude. Characteristics should be checked during a wings level stall and in a 30-degree banked turn.

On the second question: Yes, compressibility definitely affects stall speed and stalling characteristics at high altitudes.

[Mad (Flt) Scientist]

However ... the absolute max approved altitude may be impractical for test for a variety of reasons, including things like the weight of an instrumented test aircraft compared to the minimum production weight, and what that implied for altitude capability. Therefore, there is some latitude in practice, provided you can justify that the altitude you have tested to is adequate to address the case for the maximum altitude. (AC25-7C - or any other AC for that matter - is, after all, advisory and doesn't have the full force of regulation)

And to the second, there are cases where compressibility actually affects the stall behaviour at low altitude as well, although the mechanisms can be a bit complex.

[actus reus]

Even if you start the stall demonstration at Max Cruise Alt (not service ceiling), the 'one knot per second' decel will mean the one g 'break' will occur at a somewhat lower altitude.

30 degree bank turns or 'wind up' stalls can go awry very quickly and need some caution.

The test instrument to watch is the digital readout of the loading on the tail plane.

Horror stories abound.

[Gysbreght]

Gentlemen,

The original question was:

"Are transport category aircraft stalled at high altitudes in certification testing?"

[hawk37]

Great answers guys, keep the info coming, a very interesting topic. Its really amazing how fast the Prune experts respond!!

Hawk

[Owain Glyndwr]

Are transport category aircraft stalled at high altitude during certification testing?

Yes

Is compressilibilty a factor affecting stall speed?

Yes, especially when stall is defined by the buffet level at the cockpit

Are engines likely to flame out in the "thin air"?

Stall is defined in terms of EAS which is the same amount of mass flow through the engine at any altitude

OG

[Volume]

Stall is defined in terms of EAS Stall is mainly misunderstood to be defined by Airspeed...
Is compressilibilty a factor affecting stall speed?
Yes, especially when stall is defined by the buffet level at the cockpit
But compressibility is not the only factor influenced by high altitude / low air density. Handling characteristics, especially aerodynamic dampening is significantly reduced, which makes the recovery a bit more tricky. Test pilots may not realize, as they are used to it. Commercial pilots may be quite surprised by it.

The original question was:
"Are transport category aircraft stalled at high altitudes in certification testing?" The answer is: yes, but not to the same extent as at lower altitude. Most stall issues are encountered when flying in the low speed regime, during approach and landing. A lot of stall testing is done with high lift devices in use, because this is when you get closest to the margins.

If simple stall tests at high altitude do not show any anormaly, you do not further concentrate on it. Differend general handling properties at high altitude with a higher risk of overcontrolling or PIO are not seen as a stall behavior anormally and not forther evaluated, as they are simply normal. But not trained... The same applies to the engines at high altitude, they do behave differently, but that is perfectly normal, well known and not a stall issue.

Actually additional buffeting at high altitude due to compressibility effects often helps to identify a stall earlier, making it generally "easier" to handle. And pilots should be less afraid to reduce pitch / lower the nose if there is 40.000 ft of free airspace below...

[Gysbreght]

Actually additional buffeting at high altitude due to compressibility effects often helps to identify a stall earlier, making it generally "easier" to handle.

OTOH, a natural pitch-down that may be present at low altitude, is usually absent at high altitude.

Also the stall itself (maximum lift, loss of controllability) occurs "earlier", at a lower angle of attack than at low altitude.

Due to the low air density, to gain a certain increment in terms of CAS/EAS in the recovery, requires a longer time of acceleration in terms of TAS, and consequently greater height loss.

[actus reus]

'Also the stall itself (maximum lift, loss of controllability) occurs "earlier", at a lower angle of attack than at low altitude.'

So true.

Many pilots that I have asked still think that in the cruise at altitude the aircraft will stall at 16 degrees AoA.

The 'buffet' clue with a semi-supercritical wing; another thing again.

[Owain Glyndwr]

Stall is mainly misundertood to be defined by airspeed

Yeah, OK, sloppy wording on my part.
I am well aware that stall is defined by incidence and that the critical value depends on Mach number - not much up to about 0.4 or 0.5 but a steady fall thereafter.

I was however addressing the question of engine behaviour.

Certification stalls are done at idle power, which effectively defines a non-dimensional mass flow demand. Per unit cross sectional area of the entry stream tube this quantity gets bigger as Mach increases, so the intake area required decreases.

The intake is carefully sized to supply cruise thrust requirement.

At altitude, or indeed almost everywhere, at idle the engine demand is far smaller than the potential intake supply, so the principal differences between high and low altitude stalls in this regard is the amount of intake spillage.
Flow distortion due to incidence is another matter, but hey, we aren't talking big numbers here. At 0.65M stall might be about 9 deg

A lot of stall testing is done with high lift devices in use, because this is where you get closest to the margins

Well actually, the g margin retained in cruise is 0.4 incremental ( to buffet onset) whereas in approach (1.3 Vs) it is 0.69.
For me at lesst the emphasis on high lift stall testing is that small margins not cashed can have a large effect on certificated field performance and sales potential

Sorry if this sounds carping - bit liverish today!

OG

[lomapaseo]

Are engines likely to flame out in the "thin air"?

The engines are affected by the inlet separation in an aircraft stall. Too much and the engines themselves will stall/surge (but not flameout)

In the freighter DC8 crash in West Virginia, doing en-route stall testing, the crew used the sounds of engine stall/surge as a marker of the planes attitude at the time (the details are not important to this thread so I don't wish to divert the discussion)

also many reports of engines on fire, from witnesses, just before a planes smack into the ground are the results of inlet separation from wing stalls. MD80 etc.

[Owain Glyndwr]

Be nice to know approx aoa and slat/flap settings though.
OG

[flyingchanges]

Be nice to know approx aoa and slat/flap settings though.

Retracted...

[khorton]

Even if you start the stall demonstration at Max Cruise Alt (not service ceiling), the 'one knot per second' decel will mean the one g 'break' will occur at a somewhat lower altitude.
Actually, a deceleration of 1 kt/s IAS is 2 kt/s TAS (or more) at high altitude. Kinetic energy is a function of TAS^2. Getting rid of more than 2 kt/s of TAS at high speed often requires a climb for the first part of the stall test, even at idle. I've flown high altitude stalls where the stall occurred at well above the point where the deceleration was started (e.g. Global Express).

[Owain Glyndwr]

@flyingchanges

OK, thanks. About 16 deg aoa?

OG

[actus reus]

Energy transfer is also a function of mass. The TAS/IAS you have to wash off is a function of how much above Min Drag (or whatever it is called in the particular aeroplane) before you start into the low speed regime.

Level flight followed by the trend vector on the PFD the same length as 10 knots seems to work without having to do a climb. It does not compromise the test point.

Start fast and then climb above the level flight altitude (particularly if you are near to max altitude for the weight/config/CG) to start a '1 knot per second' decel obviously works for that programme but that is judgement decision.

[riff_raff]

"flame out" in a modern turbofan engine is a highly unlikely occurrence. Flame out only occurs when there is significant instability in the combustion chamber. Modern turbofan engines with digital fuel controls are able to respond quickly enough to changes in mass airflow to prevent flame outs.

[khorton]

Energy transfer is also a function of mass. The TAS/IAS you have to wash off is a function of how much above Min Drag (or whatever it is called in the particular aeroplane) before you start into the low speed regime.Mass appears the formulae for both kinetic energy and potential energy. The mass terms cancel out when you look at the vertical speed needed to get rid of KE at a particular rate. As you note, airframe drag is a huge variable. Engine thrust is also an important variable, as many engines have quite high idle settings at high altitude.