Article: Wake induced roll

[Wilfred]

Article: Wake induced roll

 

A recent Network story facetiously warned pilots to <STRONG title=http://www.aero-news.net/news/commair.cfm?ContentBlockID=635a2db5-f38b-4939-bd97-921900a15583&Dynamic=1>"beware the wrath of Dreamlifter"[/b].In one of the superfreighter's recent headline-making events, a Cessna 172 encountered the Boeing's wake turbulence, "rolled almost instantly to a 90 degree right bank, and descended in a nearly straight nose down attitude. The flight instructor took control of the aircraft, and was able to complete a recovery approximately 150 feet above the... water..."

 

Wake-induced roll

 

The usual hazard of encountering wake turbulence is induced rolling moments exceeding the roll-control authority of the aircraft. According to the FAA, the capability of an aircraft to counteract wake-induced roll "primarily depends on the wingspan and counter-control responsiveness of the encountering aircraft."

 

If the airplane caught up in another's wake has a wingspan greater than the diameter of the wake rotation, then the encountering airplane will usually be able to ride out the wake without loss of control. This is primarily why an MD-80 crew, for instance, doesn't worry too much about wake turbulence from a Cessna Citation, while the pilot of a Piper Archer behind that Cessna needs to work to avoid what for him/her may be an unrecoverable wake-induced roll. Sayeth the FAA: "It is more difficult for aircraft with short wingspan (relative to the generating aircraft) to counter the imposed roll induced by vortex flow. Pilots of short span aircraft, even of the high performance type, must be especially alert to vortex encounters."

 

Really unusual attitudes

 

Attempting to recover from wake-induced roll with opposite bank input (the natural impulse) may overstress the airplane and lead to disaster. Beyond 70 degrees of bank or so, or with a very fast rate of roll, it may be safer to recover by deflecting controls in the direction of the roll, and rolling all the way around to an upright position. This is where training really pays off. I took an unusual attitudes and emergencies course several years ago at Delta Aviation in Stillwater, OK. After an introduction to maneuvers including rolls (in an aerobatic-certified Decathalon, and wearing parachutes as required by regulation), instructor Fred de Lacerta presented in-flight simulations of wake-induced roll and showed how a simple aileron roll, going with the flow, allowed a pullout with very little g-loading.

 

It's becoming quite common for corporate flight departments and ab initio commercial pilot training programs to include "emergency aerobatics" courses in their syllabi.

 

Consider investing in emergency aerobatics course to add tricks like wake-turbulence escape to repertoire.