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INTRODUCTION
In various climates around the world, airframe icing is a silent killer. While the effects of airframe icing can be taught in a textbook and in other theoretical mediums, it is seldom completely understood until a pilot experiences it for the first time. Not only can airframe icing cause loss of altitude or aircraft speed, but it can also cause the loss of life if not reacted to properly. I call it a silent killer because that is exactly what it is. It can occur in unexpected weather conditions and extremely quickly as alluded to in the forum (Commercial incidents). This lesson merely serves to educate those of you who have not really encountered icing in an operational context. The lesson is created from personal experience, a lesson I learned that VERY nearly cost me my life. The lesson will not go so far as to talk about the various deicing or anti-icing systems available in the modern era, but will describe what airframe icing is, what it can do to your aircraft, how it is encountered, what you can do to avoid it and finally how to deal with it if you are unfortunate enough to have accumulated it on your aircraft.
WHAT IS AIRFRAME ICING?
To start the lesson, I will describe what airframe icing is. Airframe icing is the build up of ice on the airframe surface. A common mistaken belief is that airframe icing only accumulates on the leading edges of the wing and tailplane. This is grossly inaccurate. Airframe icing can also build up on the propeller, windscreen, aerials and air intakes. However it can be especially dangerous when formed on the leading edges of the wing, tail-planes or over the air intakes.
Airframe icing is hazardous for more than one reason. Some say that it adds to the overall weight of the aircraft which means that more power is required to overcome the increased weight. This is true, however not the main problem with airframe icing. The increase in weight is small compared to the adverse effect it has on the smooth airflow over the surface. The main problem therefore is that it disrupts the laminar flow of air over the surfaces. This has major ramifications for air safety!
SO WHAT CAN ICE DO TO MY AIRCRAFT?
If to much ice is allowed to build up on your airframe surfaces during flight it can have absolutely catastrophic consequences. For example, ice can cause loss of altitude, wing drop stalls, and even engine failures. I will explain these three serious consequences one by one.
Loss of altitude….
I recall an incident on a Spring season night flight where a Fokker F27 aircraft was unable to maintain 10,000 feet amsl. This is an extreme case as the F27 was equipped with the correct de/anti icing gear and is a turbo prop aircraft. Highly experienced pilots were caught out by the rapid onset of the ice accretion, which proves that even if you are experienced and have the appropriate equipment, you can still fall victim to the effects of airframe icing.
The reason this occurs is relatively simple to understand and is not really due to the increased weight of the ice. The cause is due to the following factors:
1 Reduced lift caused by the break up of the laminar flow of air over the leading edges.
2 Increased power applied to counter the reduced lift.
3 Available power reaches a ceiling and angle of attack increases.
4 Eventually the lift decreases and the aircraft sinks as there is no more power available.
Remember that this can cause the inexperienced pilot to raise the nose (when trying to stop the sink) which can lead to a fully developed stall situation!!!
Stalls…
The wing will ordinarily stall at a lower angle of attack, and thus a higher airspeed when contaminated with ice. Even small amounts of ice will have an effect, and if the ice is rough, it can be a large effect. Thus an increase in approach speed is advisable if ice remains on the wings. How much of an increase depends on both the aircraft type and amount of ice. Stall characteristics of an aircraft with ice-contaminated wings will be degraded, and serious roll control problems are not unusual. The ice accretion may be asymmetric between the two wings. Also, the outer part of a wing, which is ordinarily thinner and thus a better collector of ice, may stall first rather than last.
The stall is particularly serious when in icing conditions as it is possible that it may lead to a collision with terrain, with a fatal outcome!
Aircraft tail stall (video embedded from You Tube on 28 September 2010)
Engine failure
Ice can cause engine failure in severe conditions and prolonged exposure. This is not only limited to normally aspirated engines, but can also apply to turbo fans. Refer to the link below for a more in-depth explanation as to how this can occur.
//http://www.allbusiness.com/transportation/air-transportation-airlines/14581314-1.html
HOW IS ICE ENCOUNTERED?
This video is entertaining… and a bit light hearted, however this is the basic process for the build up of ice on your airframe.
Aircraft animation - Airframe icing (video embedded from You Tube on 28 September 2010)
You will notice three things in this video.
1. The aircraft was clean when flying in clear conditions (Visual Meteorological Conditions)
2. The aircraft flew through cloud (Instrument Meteorological Conditions)
3. The aircraft emerged covered in ice!
This is as simple as it gets to illustrate the build up of ice.
Ice forms when supercooled droplets of water are suspended in air. They attach themselves to the aircraft when they make contact in circumstances such as flight through cloud. Icing will only occur however, in certain temperatures and conditions.
The table below outlines the required temperatures.
Cumulus Clouds |
Stratiform Clouds |
Rain and Drizzle |
|
High |
0o to -20oC |
0o to -15oC |
0oC and below |
Medium |
-20o to -40oC |
-15o to -30oC |
|
Low |
< than -40oC |
< than -30oC |
Civil Aviation Authority, 2000
HOW CAN ICE ACCRETION BE AVOIDED?
Normally the aircraft will be well equipped to deal with any ice accretion, however there are instances where the build up is much faster than originally anticipated. This can cause the de-icing equipment to fall behind and then fail in its effectiveness.
There are a few sure fire ways to avoid the build up of ice to dangerous levels.
1 Ensure as pilot in command of an aircraft that you obtain accurate and recent meteorological reports for the intended route as well as alternative aerodromes.
2 Avoid routes that take the aircraft into known icing conditions for extended periods of times.
3 Choose alternative routes to the main route so that you are able to escape troublesome icing conditions.
4 Continuously monitor the build up on leading edges, remember, if there is build up on the windshield then it is probable that ice is building up on the leading edges too.
5 Maintain situational awareness at all times.
6 Ensure that your aircraft is fitted with the correct de/anti icing equipment for flight into known icing conditions.
HOW DO I DEAL WITH ICE ACCRETION?
Many pilots and multi-crews have experienced an over build up of ice on the aircraft during flight. It is not limited to inexperienced pilots as I have personally witnessed veteran pilots struggling to maintain altitude as a result of ice accretion.
There are certain tactics that should be employed once you notice ice building up on your airframe. However none are better than AVOIDANCE!!!!
The following steps should assist in dealing with ice accretion….
1 Immediately use your de/anti icing gear appropriately
2 Monitor the rate of ice accretion carefully
3 Seek to descend either visually or under radar vectors
4 Remain clear of Instrument Meteorological Conditions (IMC)
5 Land as soon as possible and either do not get airborne again until conditions improve or choose another route!!
Want to know more?
- item
- Vector Magazine - July / August 2002 // http://www.caa.govt.nz/.../Vector/Vector_2006_Issue-4_Jul-Aug.pdf
:item : AOPA Air safety notice // http://www.aopa.org/asf/publications/advisors.html
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Authors / Editors
Aircop October 2010
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