Ask Captain Lim

Dear Captain Lim,

I stumbled across your site and have read a lot of your responses. Thank you for taking your time to respond to the queries. There are many wary air travelers out there, and I am one of them. After reading your FAQ, I feel more assured in an aircraft as regards to noises, turbulence and its construction.

However, could you elaborate a little on a couple of things on air turbulence or air pockets?

Is it possible for an aircraft to fall (or rise) several hundred feet? If so, what kinds of stresses are exerted on the wings? In your previous responses I understand that the wings may bend up to 24 feet before it snaps. What are the stresses values that the wing is designed to take?

Secondly, I have just learned from your site that there are deicing mechanisms on the plane. That certainly eases my mind. I observe that during extreme winter weather, planes are deiced prior to takeoff. Is this a required safety procedure? What types of fluid are used? Does this liquid freeze? How long can an aircraft sit on the runway before this process needs to be repeated?

Can ice form on the plane if the process is not carried out and ultimately bring it down?

Jeff Davidson

Hi Jeff,

I have answered questions on turbulence (or incorrectly referred to as air pockets) many times before. Generally, they do not cause the aircraft to drop several hundred of feet, and certainly, severe (not extreme) turbulence will not dislodge a wing or knock you from the sky! Even if there is a slight drop in altitude (in the updraft or downdraft), airplanes are inherently stable, always wanting to return to its original position in space. I cannot quantify the amount of stress that would be exerted on the wing before it gives way, but I believe FAA have very stringent regulatory limitations for airplane designers to follow.

Just like you have to worry about driving in wintry conditions, (chains for your tires, driving slowly, etc,) pilots have similar worries too. To deice or not to, pilots would have to look for ice deposit on the surface of the wings, engine inlets, etc. On the Boeing 777, any ice accumulation thicker than one eighth of an inch (3 mm) must be deiced. Yes, this is a safety procedure and the onus is on the captain to deice the plane prior to take off after the consultation with the ground engineer.

The deicing procedure is either carried out at the parking bay or near the take off point (Munich). The spray for the ground deicing is a heated combination of glycol alcohol and water. The ratio of this combination and its effectiveness varies with the weather and outside air temperature. The colder it is, the shorter the duration of its efficacy.

How long can an aircraft sit on the runway before this process needs to be repeated? This depends on the type and combination of the fluid used. For instance, if it were 30 minutes, the pilot must take off within this time, or else he must return for another deicing procedure when the fluid refreezes. It has happened many times before and this is one of the causes of long delays in many airports during the severe winter months.

Icing has been cited as the cause of the American Eagle ATR turboprop crash during a holding pattern over Illinois, USA. Another ice-related accident involved an Air Florida Boeing 737-200 at Washington, DC, in 1982. In this case, the NTSB determined that the probable cause was the flight crew's failure to use engine anti-icing during the ground operation and take off. Contributing to the accident was the prolonged ground delay time (50 minutes) between de-icing and the take off during which the aircraft was exposed to continual rain.

So far, cases of planes going down due to icings have been pleasantly rare.