Colgan 3407 Down in Buffalo

[VF45]

If you cob the power with full nose up trim, will the Q400 pitch up?

 

[glasspilot1]

Its automatic, it sequences from the wings and tail. Works well, there is a knob to select Fast or Slow in the cycling.

Thanks for the reply Surf.

 

[surplus1]

The part i dont get, is if there were no pitch control input changes prior to the AP disconnect, why would it pitch up? They were already trimmed for level flight (pitch) and possibly coming out of the initial intercept turn (roll), but still 'in level trim', so if the roll excedence were to cause the disconnect, it seems that the pitch shouldnt change? I think i understand when you say that when they were slowing the trim was compensating for the required trim until it reached a limit but usually when you get a AP disconnect because of limits, the plane stays where it was, whether it be level pitch, or in a 20 degree bank turn, or what have you because thats where it was when it disconnected.. Granted you have to take immediate control to prevent something from happening further. You might be right about the wing stall but the nose would dive and the pilot would yank bank which would address the pitch up reports.

As I see it, the key to your statement is in the first sentence: "The part i dont get, is if there were no pitch control input changes prior to the AP disconnect, why would it pitch up?"

Actually there were lots of pitch input changes - all made by the auto pilot and unobserved by the crew. [If the darn airplane had a trim wheel they might have noticed it turning] Additionally, the autopilot was struggling to keep the wing from increasing the bank angle (due to the ice on that wing), which they also did not know.

The aircraft was in a descent - through 6000, 4000 to 2300. Power most likely retarded for the descent. Altitude is pre-selected to 2300.

As we reach 2300, altitude captures and is held by the autopilot. What happens now to airspeed? Unless power is added, airspeed will decrease.

If the autopilot was NOT engaged, and you didn't touch the yoke - what would happen to the nose as the airspeed decreases? It would drop off wouldn't it? If you did not want that to happen but you still wanted to slow down. what would you do? Trim nose up.

Now put the autopilot back in the equation. You told it to hold 2300 ft, and it does. As the airspeed decreases how does it do that? It trims the elevator nose up, to hold the altitude.

If you do nothing else it will continue to trim nose up and the airspeed will continue to decrease - until one of the two reaches the level required to hold that altitude with that amount of power. In other words, if the power available matches the power required to hold altitude at that speed, the auto pilot will stop triming and you will be in level flight with x amount of nose up trim and y amount of power - stable. If the power available/used remains less than required - airspeed will continue to deteriorate slowly until the auto pilot runs out of available trim capability or you exceed the critical AOA and stall.

I am only saying how it works - not what they did. I assume that they added enough power to hold the desired speed (or what they thought would hold it).

Everything is now stable and the auto pilot has stopped triming nose up -but - it has NOT removed much if any of the trim previously input. You're still at 2300 ft and headed for the localizer. Glide slope is not alive and has not captured.

As you approach the localizer the autopilot begins a turn to capture it. The auto pilot is still flying. What happens now?

As the bank increases more back pressure is required to hold the altitude. The auto pilot provides in by additional nose-up elevator trim. Something else also happens - the airspeed begins to deteriorate (slow) again. You decide to accept the slightly lower airspeed and you don't add more power - it's already where you think you want it.

Meanwhile bank angle is increasing and elevator is trimming nose-up (slowly). Bank angle tries to increase beyond 25 deg. (due to the ice) until the auto pilot exceeds its limit and kicks off. The nose pitches up and the wing stalls.

Instantly the bank increases from 25 to 45 degrees - further increasing the stall speed. What happens to stall speed as bank angle increases? Yep, its higher than in was before. It was already on the edge - you just didn't know it.

Just as you act to correct the bank the wing stalls. You call for full power and all that up elevator trim is still there. Where does the nose go before you can stop it? All the way to +31 degrees nose up and you have the yoke full forward. There's a lot of power there and some very big props.

Down goes the nose - very rapidly. Your control input has taken effect There is still too much elevator trim for that amount of power and you're pushing hard.

The instant you see 45 degrees nose down you reverse the pressure and pull like hell. You still have almost full aileron cranked in to pick up the left wing. You havent thought at all about trim. Who would?

Just as you apply that back pressure you now enter an accelerated secondary stall. The aircraft snaps to the right - exceeding the vertical bank angle. It is completely out of control - plus you just lost a thousand feet.

The rest ...............

It's all hypothetical ... just a theory.

 

[surplus1]

If you cob the power with full nose up trim, will the Q400 pitch up?

I've never flown it so I don't really know.

But, if it reacts like other propeller airplanes that I've flown the answer is yes. It would nose up even if the elevator trim was neutral. It's a prop/thrust line thing. Plus it's a high wing, which ususally aggravates that tendency.

 

[trainer8]

Hi Surp,

Very good assesment--your last couple posts. I concur with your assesment, except on a couple minor points.

1) Chealander confirmed AP disconnect occurred at stick shaker/pusher and not from AP sensory loads.

"The NTSB's Chealander confirmed that the autopilot was engaged until the stick-shaker, and the stick-pusher kicked in, signaling the start of an event that could have, and did have dire consequences. He went on to say that when this happens, the autopilot automatically disengages, putting the aircraft back in the hands of the pilot."

2) I wouldn't rule out tailplane icing. Having flown this type for 8100 hours in various icing conditions including moderate icing (tennis ball size ice sculptures on the ice detection probes on an ILS into SWF), it's ability to fly in moderate ice never produced any aerodynamic instability while accreting ice. I would be very surprised if the NTSB ruled that this would be the cause.

The application of wing flap (Dash-8s have full span wing flaps) from 5 to 15 appears to have induced the upset, which fits the NASA tailplane icing flight testing scenarios. (http://www.part135.com/TailplaneIcing.html) Chealander (NTSB) also indicated that the flight crew followed all AFM procedures pertaining to flight into known icing conditions.

What seems to be conspicuous was the situational awareness of airspeed degradation to onset of stick shaker.

Overall, I think your assessment is very good.

T8

 

[belchfire]

Does anyone have the flight and duty times for the crew for the days leading up to the accident date?

The reports tell us that an attempt was made to retract the flaps (and maybe the gear). If that is true – it was mostly likely because the PF associated flap extension with the upset. In any event in stall recoveries in this category of aircraft Training often says “max power, flaps up, gear up”. Again, I have no idea what that airline teaches, so that thought is pure speculation. In any case I consider it to be essentially irrelevant in this instance.

You are doing some very good presentation but my training has been to not change configuration until the stall recovery is complete-though I am not familiar with the Dash either.

With upwards of what, around 10,000shp(?) and those big props my guess is that while application of max power is probably called for in stall recovery training that about 50-70% torque and maintaining a level flight pitch attitude would be enough to recover.

 

[surplus1]

Hi Trainer,

I've only ridden in a Dash 8/200 once in my life. EYW-MIA many years ago. I've never even seen a live Q-400; just pics.

Your points 1 and 2 are important especially # 1. I think the time line here - the exact moment of auto pilot disconnect - is very important. If it is true that the autopilot did NOT disconnect until AFTER the shaker activated, then I would modify my hypothesis to something I have not wanted to say.

That would be that the aircraft reached 2300 ft, with insuffient power to sustain level flight. It slowed gradually - with the auto pilot trimming nose up - unobserved by the flight crew until the shaker activated disconnecting the auto pilot - followed by a stall.

This would mean that the stall speed - with the ice - [as re-programmed by the crew] had increased to somewhere around 130 kts. (given that NTSB says CAS was 134 kts on the FDR) and activated the shaker disconnecting the auto pilot. It would explain the pitch up - and cause the actual aerodynamic stall. But, it would also point to what none of us want to hear = PE.

The initial roll off would simply be a by-product of the stalled wing (1st stall) and the 2nd the product of the accelerated secondary stall. This would NOT be good for the crew.

On your point #2 - I know that if there's ice on the wings there is also ice on the tail. However, the manufacturer claims that flight test data indicates that the Q-400 is NOT susceptible to tailplane stall.

Personally I'm just not ready to buy into the tailplane stall theory, although I'm very much aware that the manufacturer will launch into full CYA mode if it believes that's necessary. Manufacturers never voluntarily admit design anomalies.

If you're right about #1, then I hope I am wrong about #2.

Good to see you.

 

[LJ45]

wish the NTSB would say what the power setting was at time of stall !!! Looking more and more like they were too slow and coupled with some residual ice let the airplane stall. I think if the NTSB would just tell us what the airspeed trend was right before the upset, the power setting and the speed at which the stall warning activated we would have much better info.

 

[surplus1]

You are doing some very good presentation but my training has been to not change configuration until the stall recovery is complete-though I am not familiar with the Dash either.
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Do you train in a jet? Or something like an L-188, DC-6/7? If so your training is the same as mine has been.

My training experience in the smaller propeller airplanes has been the opposite. I didn't agree with it but I didn't get to make the rules.

 

[trainer8]

Hi Trainer,

I've only ridden in a Dash 8/200 once in my life. EYW-MIA many years ago. I've never even seen a live Q-400; just pics.

Your points 1 and 2 are important especially # 1. I think the time line here - the exact moment of auto pilot disconnect - is very important. If it is true that the autopilot did NOT disconnect until AFTER the shaker activated, then I would modify my hypothesis to something I have not wanted to say.

That would be that the aircraft reached 2300 ft, with insuffient power to sustain level flight. It slowed gradually - with the auto pilot trimming nose up - unobserved by the flight crew until the shaker activated disconnecting the auto pilot - followed by a stall.

This would mean that the stall speed - with the ice - had increased to somewhere around 130 kts. (given that NTSB says CAS was 134 kts on the FDR). It would explain the pitch up but would also point to what none of us want to hear = PE.

The initial roll off would simply be a by-product of the stalled wing (1st stall) and the 2nd the product of the accelerated secondary stall. This would NOT be good for the crew.

On your point #2 - I know that if there's ice on the wings there is also ice on the tail. However, the manufacturer claims that flight test data indicates that the Q-400 is NOT susceptible to tailplane stall.

Personally I'm just not ready to buy into the tailplane stall theory, although I'm very much aware that the manufacturer will launch into full CYA mode if it believes that's necessary. Manufacturers never voluntarily admit design anomalies.

If you're right about #1, then I hope I am wrong about #2.

Good to see you.

My assesment differs only because of my experience flying and teaching the same Type, only smaller, less powerful models.

Watching Chealander's report on Saturday, he was clear about the stick shaker induced/automatic AP disconnect. That unfortunately means the crew was unaware of airspeed degradation and suddenly had a hand-full of tempest. Q400's have 5 panel glass cockpits. Manual trim (big wheels) were changed out for electric trim for this model.

I'm sure Vref is indicated (green dot on the airspeed tape for some aircraft) in some way to the crew. Perhaps others can comment. I hope I'm wrong. But, ATC and NTSB cursery indications are level flight at 2300' approaching KLUMP (FAF) to AP disconnect.

T8