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Continental TurboProp crash inbound for Buffalo

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Continental TurboProp crash inbound for Buffalo

Old 13th Feb 2009, 21:23
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There are obviously a number of people contributing to this thread who have a limited practical experience of operating for sustained periods in moderate icing and who additionally have only a cursory knowledge of a/c anti/de-ice systems.

An a/c does not simply have a particular de-ice or anti-ice system that is switched on/off with a consistant result rather like a light bulb. Crew have to be aware of current icing conditions and when possible help alleviate the problem by altitude or route changes. In temperate latitudes ice accretion can be markedly reduced simply by climbing or descending 3000 ft. One crew may make a decision to climb or descend in cruise, the crew of a similar a/c following along behind may simply stay at their planned cruise altitude and experience problems that the first crew avoided. One crew having avoided icing by climbing may request a late descent to destination so as to avoid prolonged exposure to ice in the descent... another crew might not bother.

As for extending the boots through an auto-system that doesn't wait for any ice to build or making a decision to wait for a build up of 1/2 to one inch of ice.. this decision should be made in accordance with the a/c manufacturers advice... a/c boots operate differently on different a/c... the manufacturer carries out the tests, follow their advice.

Again, one crew may decide to continue a flight utilising the auto-pilot and another crew, correctly disengaging it and flying manually... auto-pilots disguise the onset of trim changes required to keep the a/c flying that would otherwise indicate to a crew that things are getting seriously out of hand. On disengaging the autopilot the crew can be in for a rude awakening.

Tail icing (as I posted earlier) needs to be recognised as such and the correct recovery technique applied... with tail icing column buffet will be experienced (as opposed to feeling the buffet through the airframe as with a conventional wing stall)... the column will have a tendency to move easily foward and be increasingly difficult to move aft giving rise to Pilot Induced Oscillations... if allowed to develop the column will move sharpley forward as a full stall develops... the recovery procedure is to pull back on the column (opposite of a conventional wing stall) though by this time the forces required may be so great as to prevent the crew from recovering, especially as this event is likely to occur during a configuration change at a lower level during the approach.

This crew appears to have followed the recommended procedure in 'undoing' the configeration changes that may have caused the initial upset by selecting flaps to their original position and gear up... all sadly too late to recover. The problem with icing accidents is that all too often the evidence is lost in the resulting impact/fire.

Hopefully the FDR will offer some clues... in the meantime let's all keep in mind that, as with wind shear, ice is something best (when possible) avoided.
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Old 13th Feb 2009, 21:25
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Could someone knowledgeable please help out by describing what changes occur in the forces required from the stabilizer during the change to flaps 15?

To a simpleton like me it might seem that it must briefly supply up-force instead of the normal downforce, bringing the state of its upper surface (possibly more iced) suddenly into more critical play.

Doubtless this is total garbage, but if someone can slap my hand and put me right it would help with the question "if the stabilizer is badly iced, how would the effect of this alter on lowering flaps". A question which some here have hinted may be relevant ?
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Old 13th Feb 2009, 21:28
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I'm trying to think back a few years but for anyone interested there are 2 automatic settings for the deice boots.

Each section of the boots inflates for 6 seconds and then the next inboard section inflates. Once the sequence gets to the inboard boot one side will inflate at the same time as the opposite side engine intake boot and then vice versa. On the SLOW setting there is then a pause and the whole process starts again 3 minutes after it first started. The sequence is the same for the FAST setting but the pause after the cycle is (I think) 24 seconds meaning the whole cycle lasts 1 minute before it starts again.

It is also possible to manually cycle the boots. This requires the non flying pilot to cycle through the process using a rotary selector, each boot should not be inflated for more than 6 seconds and drastically increases the work load.

fg32, there is obviously a force required on the control column during configuration changes but going from Flap 10 to Flap 15 I seem to remember it was minimal (there will obvioulsy be more current Q400 pilots out there to prove me wrong!). In fact you can take off with Flap 15 and clean up to Flap zero and still quite easily hold the out of trim forces. Haven't tried it in any kind of icing though. Any help?
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Old 13th Feb 2009, 21:32
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Please MungoP!

Flying in Norway, I can promise you, other than you have experience in ice.

And, former AE ATR captain.
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Old 13th Feb 2009, 21:32
  #125 (permalink)  
 
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Autopilot Trim

Does the autopilot on this type aircraft not trim the elevator so that there is no pitch up or down when the autopilot is disconnected?
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Old 13th Feb 2009, 21:38
  #126 (permalink)  
 
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Flaps and trim

Which way to trim when flaps are deployed depends on the aircraft. Some pitch nose up; others pitch nose down.
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Old 13th Feb 2009, 21:39
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Amazing that we get such pertinent info from NTSB so quickly. In UK it would take months for AAIB to make anything public. Jumping the gun? or not?
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Old 13th Feb 2009, 21:45
  #128 (permalink)  
 
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Empati
Please MungoP!

Flying in Norway, I can promise you, other than you have experience in ice.

And, former AE ATR captain.


Empati, I in no way was suggesting that I was an expert (who is) on icing... simply attempting to enlighten some of the theorists of how things work in the real world.. I sincerely apologise if it came across in any other way.
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Old 13th Feb 2009, 21:50
  #129 (permalink)  
 
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Autopilot and Trim

Mungo:

I confess to not being a professional pilot, and my few experences in icing, many years ago, taught me to stay far away or go commercial. But my B36TC Bonanza maintains altitude on A/P by continuously adjusting elevator trim. So in general, turning off the autopilot hands me an airplane with perfect elevator trim for its configuration, although I am always holding on tight just in case.

Are you saying that on the Dash 8 the autopilot holds attitude/altitude by brute force and does not adjust trim?

Just curious.
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Old 13th Feb 2009, 21:53
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Video I had to watch for the DH8 Typerating at Tyrolean Airways about Tail Icing/Tail Stall.
The common practice for flap extension near max flap operating speed is about the worst one can do concerning the risk for a tailstall.

especially interesting from 7'30 on....


Tailplane Icing
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Old 13th Feb 2009, 22:02
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Thanks MungoP...I had a long day I guess.

From what I remember of the NTSB briefing, they had listened to the cockpit voice recording. He stated that the crew had done their approach briefing incl. a weather/ minima brifing and had mentioned ice buildup on the a/c after anti ice was selected ON. They verbaly selected it ON. NTSB did NOT speculate on anything! I do not think they would be ready this early to give out flight data readings. But who knows!?
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Old 13th Feb 2009, 22:17
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The Q400, ATR and B737 etc are certified to fly in icing condition, with droplet size (freezing rain) up to 50 microns. After the Roslawn accident NTSB reported that day and place a size of 200 microns! Pilots obviously have no info of this inflight, other than visually checking ice buildup. I do not know if this crew was flying through freezing rain.
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Old 13th Feb 2009, 22:28
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Video I had to watch for the DH8 Typerating at Tyrolean Airways about Tail Icing/Tail Stall.
The common practice for flap extension near max flap operating speed is about the worst one can do concerning the risk for a tailstall.

especially interesting from 7'30 on....


Tailplane Icing
Scary stuff, I wonder how much of this info is passed on to operators of high-tail turboprops.

If it turns out this due to icing, maybe it's time to revisit where in the flight profile these kind of aircraft make their config changes for landing. Who wouldn't want some extra altitude when your ride unexpectedly noses over?
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Old 13th Feb 2009, 22:30
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NASA has a very good video on tailplane icing. I did my undergraduate research project on the subject (tailplane icing) and learned a great deal from NASA. Im not attributing the blame to this accident on tail ice and subsequent stall, it is however a viable theory. I think all aviators and non-flyers alike, will find the video informative and educational on the basics of aerodynamics and the affects of ice accretion (especially on the tail). Fly safe.

Tailplane Icing
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Old 13th Feb 2009, 22:38
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But my B36TC Bonanza maintains altitude on A/P by continuously adjusting elevator trim. So in general, turning off the autopilot hands me an airplane with perfect elevator trim for its configuration, although I am always holding on tight just in case.

Are you saying that on the Dash 8 the autopilot holds attitude/altitude by brute force and does not adjust trim?
Ah, yeah, that's the problem sir! If an unusual amount of trim is being required you may not know about it until the trim runs out and you're handed (via autopilot disconnect) a serious out-of-trim situation. By then it may be too late.

Better to be flying manually in icing conditions so you know exactly what direction and how much trim is required to relieve the elevator forces.
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Old 13th Feb 2009, 22:38
  #136 (permalink)  
 
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Pitch trim on flap extension

Chesty - a general rule of thumb, from moments about the a/c lateral axis would be:

High wing: flaps down, nose down.
Low wing: flaps down, nose up.

There may be lots of exceptions, esp. if the flaps do not simply add lift and drag but also apply a torque.

I agree, the NTSB seems extra fast in this case - good job.
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Old 13th Feb 2009, 22:41
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For anyone who's interested, here's a basic GA primer on flying into icing conditions. This should help answer some basic questions.

http://www.aopa.org/asf/publications/sa11.pdf

BTW, I thought the NASA map on page 3 was rather interesting for the Buffalo area.
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Old 13th Feb 2009, 22:44
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THCROZIER... in answer to your Q

The advice in icing is to disengage auto-pilot. A/P will disguise subtle 'feel-back' to a pilot that otherwise would warn him of unnatural trim settings.. in a worst case scenario the A/P will ultimately give up trying to compensate for extreme trim settings and disengage itself leaving the pilot to cope with a hopelessly out of trim and possibly unrecoverable aircraft flight attitude. When a/c flight characteristics are steadily deteriorating due to steady ice build up (extra weight/ increasing angles of attack / disrupted airflow over critical lift surfaces and possibly under extreme conditions on some aircraft, less power available) it's far better for the pilot and a/c to be in touch with each other....
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Old 13th Feb 2009, 22:50
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Gegenbeispiel, yes I'm aware of that. It was sort of a rhetorical question to a very general statement by Charv89. Afterall we are talking specifically about the Q400 and I was trying to find out what kind of experience he had on this type (I have some).

Anyway. I think that every pilot from a part time PPL to a 20,000 hour old boy should watch that video. The interesting part for me was the signs of tailplance stall/icing on powered flying controls: Unusual trim settings was one point the video made and the Q400 trim indicator is a crappy little thing often hidden behind yellowed, cracked plastic which makes it hard enough to see at the best of times.
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Old 13th Feb 2009, 22:56
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Thanks Mungo and Fire, I get it now. Who knows, you may have saved my life!
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