The accident report on the loss of a drone does not contain recommendations, but it makes for required reading, as some of the findings can be related to manned aircraft. The NTSB recently issued a report (No. CHI06MA121) regarding the loss of a Predator unmanned aircraft 26 April 2006 that crashed near Nogales, Arizona. The unmanned aircraft, owned by the Customs and Border Protection (CPB) and operated as a public-use aircraft, was flown from a ground station.

Actually, there are two collocated ground stations. They are designed the same, but when used for different purposes the switching is markedly different. The set-up was ripe for a human factors fiasco.

When one console is used to control the Predator’s camera, moving the lever forward causes the iris to open wider. Moving the lever to the middle position locks the camera’s iris setting, and moving the lever aft decreases the opening.

When the console is used to control the Predator’s flight, moving the same lever forward opens the fuel valve to the engine, while moving it to the middle position closes the fuel valve to the engine, which shuts down the engine. Moving the lever to the aft position causes the propeller to feather. The Predator is capable of long flights, and at least three “pilots” rotated duty to control the drone. The accident pilot reported that shortly after he resumed control of the Predator that his screen went blank, so he switched to the other console. The Border Patrol agent manning the camera at the console moved away to make room for the pilot.

Checklist procedures require a pilot at both control stations when switching control of the Predator from one to the other console for purposes of executing the checklist.

This did not occur. Moreover, the pilot did not use the checklist when making the switch. The checklist requires the pilot to match the control positions on the new console to those on the console that had been controlling the Predator. This wasn’t done; the pilot said he was in a “hurry” and failed to do this. As a result, fuel was cut off when control was transferred, causing the Predator’s engine to quit, the airplane to lose altitude and crash.

The accident resulted from a basic lack of checklist discipline.
Another factor in the report bears mention:

“There is an audible warning when an engine failure occurs. However, the same tone is used for every warning; the sound was not distinctive for a loss of engine power. The avionics technician [who assisted in the transfer of consoles] stated that he heard the warning, but thought it was activating because they lost the Iridium satellite.”

To be sure, the pilot should have seen a loss of torque and an exhaust gas temperature warning on the heads-down display. But he didn’t, and the warning tone didn’t stimulate him to look.

Hmmm, sounds familiar. Recall the August 2005 crash of a Helios Airways B737 in Greece. The Greek investigators determined that the aircraft failed to properly pressurize, resulting in loss of consciousness and death of all aboard, and crash of the aircraft after fuel was exhausted and the autopilot could not maintain altitude (see Air Accident Digest, 1 December 2006, p. 1).

The report by the Greek Air Accident Investigation & Aviation Safety Board (AAIASB) noted that the warning horn that sounds when the cabin altitude exceeds 10,000 feet is the same horn that sounds if the airplane is not configured properly for take off. As the AAIASB report noted:

“The use of the same aural warning to signify two different situations (Takeoff Configuration and Cabin Altitude) was not consistent with good Human Factors principles. Over the last several years, numerous incidents had been reported involving confusion between the Takeoff Configuration Warning and Cabin Altitude Warning on the Boeing 737 and NASA’s ASRS [Aviation Safety Reporting System] office had alerted the manufacturer and the aviation industry … but the measures taken had been inadequate and ineffective in preventing further similar incidents and accidents.”

The AAIASB recommended something be done about this, but the Federal Aviation Administration did not elect to force a change in the warning horn.
A single warning for two different problems persists, and now has been carried over to unmanned aircraft. (For the full NTSB report on the Predator crash, see www.airsafety.com/reports/ROWS070922A.pdf)