Saturday, October 05, 2013

Airplanes Will Be Airplanes

This is an old accident, but recently at work I was asked to evaluate the suitability of the type involved for our operations and turned up this account of the accident while looking for evidence I could include in a package to support my recommendation. I didn't get very far into the article before it made me angry.

Two candidates were undergoing flight skills tests. On the day before the accident the first candidate was asked to demonstrate a stall. But the poor weather conditions - turbulence, rain and low cloud - tended to trigger the stick-pusher, and the commander pulled the stall-avoidance system circuit-breaker to prevent nuisance activation.

Now, deactivating stall protection for flight training purposes is not an automatic bad thing. You don't want the trainee to learn to ignore the warnings. A loud stall horn degrades communication in the training environment. The candidate should not become dependent on the a stall warning light to recognize the situation or on a stick pusher to effect the recovery. (A stick pusher is a device that, on detecting an impending stall, automatically applies control force to lower the nose. This simultaneously alerts the pilot to the situation and starts the recommended recovery procedure). Some measure should be taken to make damned sure that CB gets reset before the next flight, but I'm willing to give them the benefit of the doubt here.

But who does stall recovery practice in an actual aircraft in bad weather in IMC? Is this considered normal somewhere?

"The candidate found this exercise frightening as she experienced great difficulties, having to use all her available physical strength to regain normal flight with the engines on full power and in [instrument] conditions," said Norwegian investigation authority SHT.

No shit she was scared. She might have been fired had she refused, or simply marked as someone who didn't have the right stuff. Too bad she or the next candidate didn't refuse, because the next day they died doing it.

For the second candidate's test the following day - also in instrument conditions, and with stronger winds - the examiner instead requested slow flight up to the first indication of stall, and a recovery with minimum loss of altitude. The stall-avoidance circuit-breaker had not been reset.

While the commander added power and retracted flaps at the candidate's request, the pilot "lost control of attitude and airspeed". Altitude increased by 200-400ft (61-122m) and airspeed dropped to just 30kt (56km/h), even though the stall warning activated and full power was applied.

Just 37s after the control loss, and with an eventual sink rate of 10,000ft/min, the turboprop hit the sea in a near-horizontal attitude, killing all three on board.

I didn't recommend the airplane as suitable for our operations, but not because of this accident. Any airplane will stop flying if you get the angle of attack high enough and don't break the resulting stall. I have a theory that the more comfortable it is inside an airplane the easier it is to forget that you have to fly it. That doesn't stop me being happy that we've finally got the heater working well, though.


green said...

which heater you are referring to? may be it's a reference from your previous posts. :-)

Sarah said...

Fixing the heater means it's more comfortable, and isn't necessarily a reference to balky gas heaters. Though it could be.

Tail stall? Or just high control force and IMC disorientation?

Stall recovery "with a minimal loss of altitude" has been the emphasis in CPL and ATP flight test standards, and that's fine as long as the stall is recovered. I thought requiring dangerous demonstrations during flight test/checks went out with single-engine (one feathered) Vmc demonstrations.

I know nothing about the Merlin other than I like the name ... but I'd hold out for a C90 King Air.

zb said...

I have zero experience when it comes to flying airplanes, but your theory of comfort vs. forgetting to take responsibility is definitely true. I don't even have to think about AF447 - cars are convincing enough. I like driving relatively old cars and prefer a bike whenever feasible. No matter how fancy and expensive your car is, I believe that somewhere beyond 130 km/h, it boils down to some simple physics between your rubber tires and the asphalt. Seeing someone drive at or above 180 km/h with as little as 30 m to the car in front of them makes me sick. Knowing that at least the speed is even legal on some roads over here (.de) is scary.

majroj said...

I notice the cited crash eval and recommendations did not mention the disabled equipment.

Who does Quality Evals on these "reports"?

How about "criteria for disabling or masking selected instruments and warning systems will be in accordance with already-establioshed protocols and a formal marker prominently displayed "(insert location and description) "to assure the condition is returned to flightworthy" or some such.

Anonymous said...

I'm (obviously easily) confused.

Is the SHT trying to say that there is a trade off between (incomplete/unrecognized) recovery with minimum altitude loss, vs. recovery that takes more altitude but is complete? Maybe a secondary stall type situation which is less obvious with the stall horn deactivated?

Heck, after AF447 let's just ensure that pilots can do *any* stall recovery.

I guess I don't get how you can recover from a stall without lowering the nose...


A Squared said...

I guess I don't get how you can recover from a stall without lowering the nose...

Well, without intending for this to be taken as a "how to" for any airplane; "lowering the nose" is relative the horizon, a stall is relative to the airflow. An application of thrust can change an airplane's angle of motion through the air, without changing the airplane's pitch attitude relative to the horizon. With enough thrust, it would indeed be possible to power out of a stall without changing attitude. A few years back, there was an airshow performer flying a very small airplane with a very large turboprop engine, which had a thrust to weight ratio greater than 1. ie: he could actually hover the airplane pointing straight up. That's an extreme example, but that airplane would be able to power out of a stall.

Again, nothing here is to be construed as a recommended stall recovery procedure, I'm just commenting on what is within the realm of aerodynamic possibility.

A Squared said...

further on the stall recovery, typically transport category aircraft are not stalled on pilot flight tests, rather recovery is initiated at the first indication of a stall. This may be a stick shaker, stick pusher, or other warning. If recovery is initiated at the first indication, the stall really hasn't occurred yet, and it possible to recover without losing much altitude. The Merlin isn't technically a transport category aircraft, but most of the above applies. The reason a Merlin has a stick pusher, is because the stalling characteristics are rather ugly and quite a bit of altitude is lost, so a system was installed which was intended to prevent the airplane from reaching a stall.

I have read accounts of pilots who have stalled Merlins on flight tests of the aircraft (not pilot checkrides) in clear conditions, where recovery took 5,000 ft of altitude.