Pilots on more than one in 20 flights admit they suffer fatigue when they begin their descent to airports.
The study by Cathay Pacific, believed to be the most detailed of its kind into cockpit weariness, found pilots on 21 of 372 flights were at least mildly fatigued on starting their descents.
The survey measured fatigue on the Samn-Perelli scale of one to seven which ranks one as fully alert, four as a little tired, five as moderately tired, six as extremely tired and seven as exhausted.
Pilots rated their own levels of tiredness and on 21 flights gave themselves a score of between five and seven.
The survey, conducted on Cathay Pacific flights in November and December 2010, found that the flights which resulted in most fatigue were overnighters from India to Hong Kong, including ones via Bangkok.
The airline's general manager Henry Craig said in a circular to pilots that the fact that only 5.6% of flights generated a score of five or above on the scale was "very encouraging."
The survey was part of a comprehensive fatigue risk management system established by the airline in response to industry-wide concerns over the difficulties faced by crews flying multiple time zones.
"This, we believe, is the largest fatigue survey ever conducted by an international airline," a Cathay Pacific spokeswoman said.
Steps had already been taken deal with flights identified as fatigue risks and the situation was being carefully monitored, she said.
(Sapa, October 2011)