ice AOA

[ontheair 3]

what does AOA stand for in the expression "ice AOA" ? angle of attack ? sounds strange to me...not that clear for me as it's not listed in the terminology/abbreviations used in the AOM manualMerci

[Chock 17,769]

AOA is indeed 'angle of attack' This is the angle at which the wing is actually going though the air, as opposed to the climb or descent angle of the aeroplane fuselage itself, because aeroplanes often fly with their nose slightly high even when going along in level flight. The name relates to what angle the wing is at when it is slicing through (attacking) the air, thus we get the term 'angle of attack'. If you hold your hand out of a car window as it is driving along, and tilt your hand up at the wrist, the angle your hand is meeting the oncoming wind at, will be the angle of attack.On most turboprops, there are fairly specific procedures for icing conditions, because they are far more likely to cruise for long periods at around 18-20,000 feet than big jet airliners, and there are more rain clouds with ice in them at that kind of altitude, so turboprops by their very nature, are more at risk from icing than jet airliners; jets normally climb up way above that danger zone. When icing conditions are expected, the speed bugs on a turboprop are usually moved to different settings in order to reduce the AOA of the wings, since at higher speeds, the wing does not have to be tilted up so much to generate sufficient lift.When you get ice on a wing, it builds up all over the wing, but it mostly forms along the leading (front) edge of the wing. This has the effect of changing the shape of the wing. The usual result of that is that the airflow is forced to travel a bit higher up and over the top surface of the wing at the leading edge. This is important to be careful of because under normal circumstances, whenever you increase the angle of attack for a wing, the air is also forced to go a bit higher up and over the wing (which is done to generate more lift). So when there is ice on the wing, it has the effect of increasing the angle of attack because the ice coating has changed the shape of the wing.That limit of how much you can increase the angle of attack is the point where the air is forced up so high over the top of the wing that the airflow separates from the wing before meeting the trailing (back) edge of the wing; when that happens you get a stall (before that happens, you can usually feel it is coming because the airflow gets more turbulent over the wing, and that turbulent air spills back and buffets the elevators, so you feel the control column shaking a little bit as you approach a stall). So if ice on the front edge of the wing is already making the air go up a bit higher over the wing, you dare not raise the wing's angle of attack as high as you would normally be able to under non-icing conditions, since the ice is effectively adding to the wing's angle of attack, thus you are more likely to stall the wing when in icing conditions. To avoid that, you keep the angle of attack low.Also of interest...The other icing danger with turboprops in particular, is tailplane icing. The tailplane's function is to force the aircraft's tail downwards under normal circumstances, to counter the unstable effect of the main wing generating lift which would cause the aircraft to somersault up and over if there was no horizontal stabiliser forcing the back end down. To do this, the horizontal stabiliser is essentially a wing that is 'flying upside down' i.e. it is generating lift because of its shape, but its shape is such that the 'lift' it creates points downwards rather than upwards (a bit like the spoiler on a racing car). It too can ice up at the front edge and have its angle of attack altered, and the problem with that is, it can be working just fine even when iced up a bit, but if you lower the flaps, the air is forced downwards at the back of the wing where it comes off the flaps, and that air will then meet the tailplane at a much higher angle of attack, which can make the tailplane stall, and if that happens, the elevators get sucked into the vacuum under the stalled tailplane, which will force the aircraft into a nosedive. This is one of the main dangers of icing on an aeroplane, and especially on a turboprop, because they spend a long time flying about in weather that is more likely to present icing conditions.To counter the effects of icing, turboprops have a few things available to them: De-icing boots - these are rubber bags mounted along the leading edges of the wings and tail which can be inflated with pulses of air to crack the ice off the leading edge of the wing. Heating elements - these can either be actual electric heating strips mounted in the wings and tail, or warm air from the engines that is ducted to the areas where icing is likely. Beyond these protection systems, what also helps is to descend into warmer air at a lower altitude, which will also melt and crack the ice off the wings and tail given enough time.Al

[scandinavian13 5,192]

Most comprehensive post of the day. Thanks for including the tail icing bit. Most people forget about that.Also, keep in mind that the general rule of thumb for icing is visible moisture and +8 to -8C. That's possible even in the summer. Last night I was cruising along at 16,000' and ran through the back end of those East Coast storms, picking up some ice (even though it was 32 on the ground at CRW). At 16,000', it was -1C, so it was either down to 12,000 (assuming the standard lapse of 2C/1000') or up to FL200, to get out of that +/-8C range. Since I was close enough to the field, I brought it down to 12 and went in from there.

[Houghton11 12]

Hehe very authoritative explanation Al!In the J41, it basically adjusts the stall warning systems and other associated equipment to operate at a lower angle of attack then what the aircraft would stall at without ice.The J41 isn't as susceptible to tailplane icing or stalling as other similar aircraft is. Aircraft with high mounted wings and big flaps can severely disrupt the flow around a tailplane surface, increasings it's critical angle of attack. The J41's low-wing design keeps the horizontal stab in relatively clean air.The problem is, the recovery for a tailplane stall is backwards from a typical stall - you pull the stick back sharply, not forward...so it's something that isn't touched upon much.Good video on the whole thing:

http://video.google.com/videoplay?docid=2238323060735779946#

Important quote from Professional Pilot Magazine: "Stall symptoms may not be detected by the pilot if the autopilot is engaged"

[ontheair 3]

Thank you Al and all !"ice AOA" is now crystal-clear "Good video on the whole thing" I"m afraid the link seems to be corrupted or broken

[Houghton11 12]

"Good video on the whole thing" I"m afraid the link seems to be corrupted or broken

Wierd.Here ya go! :http://video.google.com/videoplay?docid=2238323060735779946#

[driver101 26]

Hehe very authoritative explanation Al!In the J41, it basically adjusts the stall warning systems and other associated equipment to operate at a lower angle of attack then what the aircraft would stall at without ice.The J41 isn't as susceptible to tailplane icing or stalling as other similar aircraft is. Aircraft with high mounted wings and big flaps can severely disrupt the flow around a tailplane surface, increasings it's critical angle of attack. The J41's low-wing design keeps the horizontal stab in relatively clean air.The problem is, the recovery for a tailplane stall is backwards from a typical stall - you pull the stick back sharply, not forward...so it's something that isn't touched upon much.Good video on the whole thing:

http://video.google.com/videoplay?docid=2238323060735779946#

Important quote from Professional Pilot Magazine: "Stall symptoms may not be detected by the pilot if the autopilot is engaged"

Thanks for sharing mate!This is cool stuff!!!