TSB questions standby instrument placement

 - May 27, 2008, 10:15 AM

Following the 1998 crash of a Swissair MD-11 off Peggy’s Cove, Nova Scotia, Canada’s Transportation Safety Board (TSB) has issued an aviation safety advisory concerning the location, size and power sources of standby flight instruments. However, the TSB is emphasizing that no evidence suggests these caused the accident.

Nevertheless, the Canadians point out that the emergency’s circumstances–diverting to an unfamiliar airport at night, with the probable loss of primary instruments due to electrical failure, a smoke-filled cockpit and the need for the crew to wear oxygen masks and goggles–could have been exacerbated by the size, layout, locations and power sources of the standby instruments, which the pilots may have had to use. The MD-11 debris field on the ocean floor measured roughly 350 by 300 ft, indicating a very steep impact angle.

The aircraft had small standby attitude, airspeed and altitude instruments located side by side at the bottom of the center instrument panel, ahead of the power levers. A compass at the top of the windshield, to the left of its center pillar, provided standby heading. Only the attitude indicator was required to have independent power.

The TSB observed that pilots forced to use standby instruments must quickly adjust to an unfamiliar layout outside their normal line of vision. In adverse circumstances, “The result could be disorientation of the flight crew, and loss of control.” The MD-11’s standby compass also required a considerable vertical scan to complete an instrument cross check, thereby risking coriolis illusions from large up-and-down head or eye movements during turns, which can produce severe disorientation, even among experienced pilots.

“The challenge of using standby instruments,” the TSB stated, “would be greater for flight crews not well trained in their use, or lacking recent practice.” However, realistic simulation training would include loss of other instruments, smoke in the cockpit and the wearing of oxygen masks and goggles, none of which is required under current regulations.

The TSB is now studying independent “get home” packages of standby instruments incorporating navigation and communications, and it suggests regulatory authorities
review their standby requirements, including grouping instruments together in standard layouts, and ensuring adequate pilot training in their use. Swissair MD-11s now carry combined displays of standby attitude, airspeed, altitude and heading in conventional layouts, with dedicated 45-min power supplies.