The National Transportation Safety Board has recommended that transponders in UAVs should have independent power sources so air traffic controllers can continue to track them following engine or generator failure.
The inquiry and recommendations are the first by the NTSB concerning UAVs and have come about following the investigation of an unmanned air vehicle (UAV) accident in the USA in 2006 involving a General Atomics Predator B single-engined turboprop UAV.
The UAV was being operated by the Customs and Border Protection agency for medium-altitude surveillance work in the south-west USA on 25 April 2006 when it crashed after its remote pilot inadvertently shut the engine down.
A key NTSB finding is that the vehicle became invisible to controllers responsible for providing separation between the UAV and civil traffic for several hours before the engine shutdown.
This occurred while the aircraft was supposed to be flying a "lost-link" mission profile, a system that puts the aircraft in a known, safe holding pattern until control links can be re-established.
After having its engine mistakenly shut down, the UAV descended into a sparsely populated area near Nogales, Arizona, causing major damage to the aircraft, but no injuries on the ground. The nearest house was within 100m (330ft) of the aircraft, says the NTSB.
The board reports the probable cause of the accident as the pilot's failure to use a checklist when trying to re-establish control of the Predator using a back-up control station after the primary station experienced a "lock-up", a problem that had occurred 16 times during the four months before the crash.
When the pilot used the secondary console, configuration differences led to the unintentional engine shut-down.
Approval to fly UAVs in civil airspace is granted by the US Federal Aviation Administration through a one-year certificate of operations that defines the airspace where the vehicle will fly, and any limitations on its use. The agency issued 110 certificates in 2006, and has approved 55 so far this year.
NTSB recommendations include requiring UAVs' transponders to be able to remain operational under all circumstances by providing an alternative power source, recording all conversations among UAV operators and air traffic control for use in incident and accident investigations and requiring face-to-face periodic reviews by UAV operations teams and local air traffic controllers, who would discuss issues that arise during normal and contingency operations and how to deal with them.