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737 Ice & Rain Protection

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737 Ice & Rain Protection

Old 25th Oct 2008, 16:49
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737 Ice & Rain Protection

That might be a dull question but i'll go for it:
Why isn'it previewed to anti/de-ice the horizontal stabilizer?
Thanks for the response(s).
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Old 25th Oct 2008, 18:07
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Fin not de-iced either. Something to do with Aerofoil shape means ice doesn't collect. Apparently. What is more irritating is that the outboard slat is not de-iced on the -800 which often results in a full wing de-ice just because you got ice on that bit. Ho hum.
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Old 25th Oct 2008, 18:47
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What is more irritating is that the outboard slat is not de-iced on the -800 which often results in a full wing de-ice just because you got ice on that bit. Ho hum.
- errm, wouldn't ice on that slat indicate you might have ice elsewhere on the wing?
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Old 25th Oct 2008, 20:09
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I landed a 737-300 at Stockholm after flying through some Sc for about 5 minutes. Can't remember the OAT but it was within the definition of icing conditions, so we selected ignition on and engine TAI. After 2-3 minutes we noticed ice building up on the wing and windshield wiper posts so we selected wing TAI as well. The wing TAI seemed to be doing its job but the ice on the windshield wiper posts was growing quite quickly. We started the APU and selected the ignition to 'Flight'.

After an uneventful landing, we were amazed during the subsequent walk-round to discover 4 inches (yes four inches) of ice all over the unprotected surfaces. These included the fin, stabiliser, engine spinners and radome. As the OAT was hovering around freezing we had the whole aircraft de-iced before we could go again.

So, the theory that the tail 'doesn't collect ice' is not borne out by my experience. What is remarkable, however, is that there were no handling or performance issues that worried us. Some time later I discovered that Boeing conducted tests using wooden shapes attached to the leading edges of the tail surfaces during certification, in order to show that TAI of these areas was not necessary.

By contrast, I seem to remember a Viscount that crashed due to a very small amount of ice on the tailplane causing flow separation when the flaps were fully lowered. Other types I've flown have AFM notes warning about similar problems but on the 737-300 that didn't seem to be an issue.

Currently on the 747-400 and that only has wing leading edges and engines de-iced as well.
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Old 25th Oct 2008, 20:25
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Originally Posted by eckhard
.... What is remarkable, however, is that there were no handling or performance issues that worried us. Some time later I discovered that Boeing conducted tests using wooden shapes attached to the leading edges of the tail surfaces during certification, in order to show that TAI of these areas was not necessary..
Not just Boeing. That's precisely what the regulations require you to do - if you want to be approved for flight into known icing condition you, not surprisingly, have to show that the aircraft can in fact safely fly in those conditions. So everyone does it, one way or another.

However just because a part of the aircraft is not de-iced or anti-iced does not mean it's ok to take off with ice there. All critical surfaces need to be clean for takeoff, whether they have provision for de-icing or anti-icing in flight.
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Old 26th Oct 2008, 01:27
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I think Rainboe has it. The stab is kept 'in trim' at all times and as such produces a small amount of negative 'lift'. Significant ice may form but that ice will not stall the stab.

Like Rainboe I have never experienced the ice accretions echard has described but it is comforting to know the aircraft performed well.


Regards
Exeng
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Old 26th Oct 2008, 08:04
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I think the only problem on a 737 might be the fin losing effectiveness in such conditions. The spinner problem shoud have been sorted by the profile change following the severe engine icing event on a 300 in Germany in the ?90's?.

They are, as exeng says, rare. My only experience was landing a Jet Provost after an hour in icing conditions with huge layers of ice all over it. Mind you, I tend not to look back at the old ships now-a-days as I walk away.....
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Old 26th Oct 2008, 09:04
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Thanks a lot for your inputs. Maybe it has to do with CS 25 (certification spec.). I also found a film on this website: brahimtahiri.googlepages.com. And i'm still searching....
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Old 26th Oct 2008, 21:59
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Originally Posted by exeng
I think Rainboe has it. The stab is kept 'in trim' at all times and as such produces a small amount of negative 'lift'. Significant ice may form but that ice will not stall the stab.
Actually, that's rather on the wrong track.

An all moving tail and a fixed tail will both have to generate exactly the same lift to keep the same aircraft in trim (if you were to be comparing a 737 with fixed tail and a 737 with moving tail flying side by side in the same conditions)

To generate the trim force the all moving tail has to do it by changing the angle of attack of the whole tail; the fixed tail does it by changing the camber for a fixed AoA. Generally, the achieve a given trim case the fixed tail will have a lower tailplane AoA as a consequence. Therefore one would usually expect the moving tail to be closer to the stall AoA than the fixed tail. Aircraft will all moving tails certainly can stall the tail; the first BAe Hawk prototype did precisely that when the flaps were deployed on one test flight. Part 25 aircraft are required not to stall the tail, but push them outside the cg envelope in icing conditions and you might find the tail in a stalled condition.

The Viscount accident wasn't strictly a tail stall in the sense that the whole tail stopped generating lift; rather it was a control surface snatch due to flow separation (due to ice) causing huge hinge moments on the (manual) elevator, pulling the elevator to the nose-down position despite crew force application.
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Old 26th Oct 2008, 22:03
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Originally Posted by flyby797
Thanks a lot for your inputs. Maybe it has to do with CS 25 (certification spec.). I also found a film on this website: brahimtahiri.googlepages.com. And i'm still searching....
Try this NASA web page which provides links to courses and other material. NASA Lewis/Glenn has been conducting research on icing effects for years using a dehavilland Twin Otter, up to actually intentionally inducing a tail stall (please don't try that at home!). There's supposed to be a S-3 Viking being prepared to replace the Twin Otter in the next year or two. The guys involved are all very helpful and keen to spread the word on icing, so you can try dropping them a line, I'm sure there's contact info on the site linked to above.
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Old 27th Oct 2008, 15:30
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Stockholm?
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Old 27th Oct 2008, 17:53
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Correct BOAC, 1977 near Stockholm I believe.
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Old 28th Oct 2008, 13:22
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The Viscount tailplane was de-iced by hot air ducting from the inboard engines. In the case of the accident mentioned, the aircraft had been on descent from cruise with low power on all engines. At this low power the heat rapidly disssipated during the hot air flow down to the tail area and was not hot enough to stop ice forming.

During my time on the Viscount we would keep the power well up on the inboard engines during descent in icing conditions which sometimes made the descent profile flatten but one had to avoid the temptation of further reducing power to the inboard engines because one didn't know whether the hot air was enough to ensure ice free tail surfaces.
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Old 6th Jan 2009, 20:22
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Logic?

However just because a part of the aircraft is not de-iced or anti-iced does not mean it's ok to take off with ice there. All critical surfaces need to be clean for takeoff, whether they have provision for de-icing or anti-icing in flight.
Sooo... to provoke a bit: You can fly around with thick ice on the stab, land or make a single engine go-around with ice on the stab - but you cannot take off with a thin layer of ice on the stab? What's the difference?
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Old 6th Jan 2009, 23:24
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Originally Posted by Crossunder
Sooo... to provoke a bit: You can fly around with thick ice on the stab, land or make a single engine go-around with ice on the stab - but you cannot take off with a thin layer of ice on the stab? What's the difference?
Easy-peasy that one.

You demonstrate safe flight and landing with ice accreted in-flight (for unprotected surfaces). You don't demonstrate that you can safely rotate for takeoff.

You can do certain things, because the aircraft is designed with those taken into account. You cannot do things (well, certainly you should not try to) if the OEM didn't prove it was safe first.
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Old 6th Jan 2009, 23:46
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Humbly, I add that ice accreted in-flight is "formed" by the airflow at high Q numbers; ice accreted on the ground has formed 'willy-nilly' and its shape may be quite unaerodynamic.

Not an elegant explanation, but pilot-speak.

GF
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Old 7th Jan 2009, 00:09
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Does not certification with and without a powered horizontal stabilizer/flight controls have some bearing on whether the stabilizer is de-iced or anti-iced protected? The MD80 has non powered stabilizer and it is deiced, the 737 has a powered stabilizer and has no ice protection. The corporate Challenger I flew had powered surfaces and it was not deiced, the unpowered Westwind was (I think, but that was a long time ago).
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Old 7th Jan 2009, 00:16
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Read my Post #28 at:

http://www.pprune.org/tech-log/35628...ake-off-2.html

in the B737 x-wind take-off thread in the Tech Log.

I have first hand experience with tail ice on a Boeing 737 (200 Model). Anyone who thinks or tells you that the tail on that airplane will not ice or that performance isn't affected is NUTS.

There's a real nice heater back in the tail (the APU) that could heat the tail real easy, but it'd cost a few pounds of fuel. It is just another example of cost saving measures that come into play in the manufacture of not only airplanes but many other things.
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Old 7th Jan 2009, 00:44
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It is also worth considering that the takeoff demands the greatest performance of any phase. The airplane is at the heaviest weight and is expected to lift off at a predetermined point and produce a specified climb performance.

Boeing has never protected their tails. The premise is that they have used an airfoil that produces so much margin over stall throughout the flight envelope that it simply will not be vulnerable to icing. They certainly do the testing with three inch ice shapes, but the impression I have gleaned from numerous conversations with them is that their tails have a lot of capability in reserve (they don't like to get into details).

I believe that Airbus approaches this in the same way.

Contrast this with Douglas and the DC-9/MD-80. I believe that they started with the intention of not protecting the tail as well. Unfortunately, the stabilizer did not quite make the grade with ice shapes attached. In fact, the DC-9/MD-80 does have a demonstrated problem with ice contaminated tailplane stall. I am aware of two events ...I'm sure there are many more...in which a pitch over was experienced following the selection of landing flaps (the almost universal point of initiation for these types of events). I believe that the reason that no more serious events have occurred is because, unlike a turboprop, landing flap on the jet is usually selected prior to reaching the outer marker. This leaves plenty of room for recovery. In the Viscount/Jetstream/YS-11 events, final flaps were typically selected much closer to the ground.

In any event, Douglas had to bolt on ice protection for the stabilizer, which I believe is why there is not enough energy allocated to run both the wing and the tail at the same time. The required cycling of the tail ice protection prior to final approach is extremely important.

As far as the dynamics of an ICTS event, certainly the first manifestation is a notable reduction in longitudinal stability and perhaps a vibration in the elevator controls (as opposed to an airframe shudder from wing flow separation). The elevator snatch that Mad Scientist describes is next, but a complete tail stall is possible depending on the ice shape and tail angle of attack. In any event, a retraction of the flaps reduces the downwash angle and instantly reduces the tail angle of attack. Overpowering the elevator with brute force will re-camber the tail and restore flow attachment.

To date, this has not been a problem on any control surface that uses hydraulic power for flight controls (another reason Boeing is confident). Whereas a genuine elevator snatch will jerk the control column right out of your hand and smash the panel with it, the aerodynamic forces that generate this are easily overpowered by hydraulics.
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