Can you maintain your flight level?

A recent serious loss of separation in Controlled Airspace has highlighted how vital it is that pilots inform Air Traffic Control if they are unable to accurately maintain their allocated flight level for any reason.

The pilot intended to make a short over-water flight from his home base to a destination on the south coast of England. The weather forecast for the return flight was not particularly good and was expected to preclude a VFR flight so an IFR flight plan was filed for a flight via the airways system. As the pilot was relatively inexperienced at IFR flight and with the poor weather forecast in mind he asked an experienced instructor from his local flying club to accompany him on the flight.

The flight northbound at FL50 was uneventful although at times in turbulence. During this flight the pilots noticed the suction instruments, especially the VSI and altimeter, were very lively which they believed suggested some form of static source blockage. The return flight later that day was made at FL60. This was just at the freezing level necessitating the use of pitot heat and involved flying in and out of the cloud tops. It became increasingly turbulent and the instructor asked the pilot if he required any assistance. As the pilot felt that he needed the practice at IFR flight under difficult conditions he elected to continue unassisted, but with the instructor keeping a close eye on his flying. ATC then gave a frequency change and as the pilot was changing frequency and retuning the VOR receiver the aircraft entered an area of worsening turbulence. The VSI indicated a 1500fpm climb and the altimeter was indicating a climb also and so the instructor reminded the pilot to maintain his altitude by lowering the nose to regain FL60. The turbulence and the erroneous readings of the instruments made it difficult to control the aircraft accurately and the pilot over-controlled the aircraft, entering a high rate of descent. The instructor then took control of the aircraft and climbed back to FL60 where it flew into clearer air and the flight continued at FL60.

However, the descent had taken the aircraft into confliction with opposite direction traffic on the airway below and, as both aircraft were flying in IMC, neither saw the other. The controller believed both aircraft were maintaining levels separated by 1000 feet and therefore not in conflict. He was busy dealing with other traffic and did not notice the level deviation so was unable to give avoiding action or traffic information.

Had either the pilot or instructor mentioned to ATC the difficulty the turbulence was causing them in maintaining level flight, the controller could have vectored the aircraft in order to provide lateral separation from the conflicting traffic or provided increased vertical separation.

When flying in controlled airspace, please always inform ATC if you are having difficulty maintaining level flight for any reason.

July 2006
Paul Hodgson
ATC Investigations
London Area Control Centre

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