Accident investigators at Canada’s Transportation Safety Board have issued an aviation safety advisory to international aviation authorities and aircraft operators concerning standby flight instruments. While aimed at the use of standby flight instruments in the McDonnell Douglas (now Boeing) MD-11, the advisory is applicable to virtually all aircraft in which these instruments are installed. Under certain conditions, the advisory warns, the use of standby flight instruments could result in "disorientation of the flight crew and loss of control."
The board’s advisory stemmed from its investigation of the Swissair MD-11 crash in the Atlantic Ocean off Nova Scotia on Sept. 2, 1998. And, although the board’s chief investigator specifically avoided placing blame on the standby instruments as the direct cause of the crash, he clearly felt that the issue of an aviation safety advisory should not be delayed until the publication of the final accident report, expected in mid-2002.
‘Pan, Pan, Pan’
In the accident, the Swissair aircraft was cruising at 33,000 feet over the Atlantic, en route from New York to Geneva, when the crew smelled an unusual odor on the flight deck. Three minutes later, the crew transmitted a message, prefixed by the international "Pan, Pan, Pan," notification of an urgent onboard condition. The alert, short of a Mayday, went to the Moncton, New Brunswick, area control center, and it requested an immediate diversion to the nearest airport. The crew headed toward Halifax International, just 70 miles northeast of the aircraft’s location.
But the MD-11 was still too high to make a straight-in approach, so the crew turned away to make a descending racetrack procedure to dump fuel over the ocean while losing altitude. At this point, the autopilot had dropped out and various other system-related failures had occurred. Some affected the primary instrument displays.
Almost as soon as the aircraft turned onto the racetrack and commenced dumping fuel, the captain declared a full emergency and stated he had to land immediately. This was the MD-11 crew’s last communication. Approximately six minutes later, and some 17 minutes from their initial "Pan, Pan, Pan" call, the aircraft struck the water. The MD-11 disintegrated on impact, and all 229 passengers and crew were killed. The major debris field on the ocean floor was reported shortly after the accident as measuring roughly 350 by 300 feet (107 by 91 meters), indicating a very steep impact angle.
Post-accident analysis showed that all onboard electrical power was lost at about 10,000 feet, causing both the flight data recorder (FDR) and the cockpit voice recorder (CVR) to cease operating. All instruments and displays requiring electrical power stopped, as well. As the advisory notes, "...the workload of the flight crew was substantial. They were faced with diverting to an unfamiliar airport, at night, with smoke in the cockpit and with oxygen masks on."
A Potential Hazard
Whether or not the use of standby instruments for emergency guidance contributed
to the Swissair crash, the Canadian Safety Board clearly feels there could be a hazard. In the MD-11, small standby attitude, airspeed and altitude instruments were located side by side at the bottom of the center instrument panel, above the power levers. A retractable compass at the top of the windshield, to the left of its center pillar, provided standby heading.
Under regulations, only the attitude indicator was required to have independent power. Canada’s advisory suggests that this should be changed to apply to all standby instruments. In addition, the MD-11’s standby compass 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. (Webster describes the Coriolis effect, named after the 19th century French mathematician and engineer, Gustave-Gaspard Coriolis, as an "apparent deflection, caused by the rotation of the earth, of a body moving relative to the earth.") These movements can produce severe disorientation, even among experienced pilots.
The board observes that pilots forced to use standby instruments must quickly adjust to an unfamiliar layout, outside the normal line of vision. In adverse circumstances, the board states, "The result could be disorientation of the flight crew and loss of control."
The advisory cautions: "The challenge of using standby instruments would be greater for flight crews not well trained in their use or lacking recent practice." And, while the board notes that incidents in which pilots were forced to use standby instruments as their sole means of obtaining attitude, direction and airspeed were rare, pilots who had to rely on them in emergency situations had noted deficiencies such as:
Regarding the last deficiency, the board added that realistic simulation training would have to include loss of the primary instruments, smoke in the cockpit and the wearing of oxygen masks and goggles, none of which is required under current training regulations.
The Canadians are studying the development of independent "get home" packages of standby instruments plus navigation and communications. They propose that international regulatory authorities review their standby requirements, including grouping instruments together in standard layouts and ensuring adequate pilot training in their use.
After the 1998 accident, Swissair MD-11s were equipped with combined displays of standby attitude, airspeed, altitude and heading in conventional, "Basic T" layouts. Additionally, the instruments have dedicated, 45-minute power supplies. Swissair flight crews also are undergoing more realistic training in the use of standby instruments in adverse conditions. However, it is not known whether other operators of the MD-11 or other aircraft types are doing the same.
The Federal Aviation Administration (FAA) and other international regulatory agencies have yet to amend their current rules concerning standby instruments. But in the wake of Canada’s aviation safety advisory, aircraft manufacturers, equipment installers and regulatory agencies probably will be looking at the future positioning and use of these units in a different, and much more critical, light.