Smoke in the air conditioning system may be the biggest distractor from more serious problems in the aircraft electrical system. If smoke in the A/C system can be prevented, and yet smoke persists, it's a fair bet that the smoke is coming from electrical arcing or similar electrical system problems and it is probably prudent to get the airplane on the ground.

A new fan bearing vibration monitor developed by Inflight Warning Systems (IWS) of Yorba Linda, California detects failing fan rotor bearings, shuts the fan down, and notifies the flight crew before catastrophic bearing failure leads to rotor contact with the housing. Using this system tells the crew that if smoke persists, it must be from another source.

Swissair 111, which crashed in 1998 from a runaway electrical fire, might have been helped by a system such as this. In the Swissair case the flight crew noticed an abnormal odor in the cockpit, concluded it was from the A/C system, and elected to divert and dump fuel. By the time they realized it wasn't a garden variety A/C problem, the fire had reached the cockpit. Had they known it was not an A/C problem they might have acted in greater haste to get the aircraft on the ground.

The monitor is a small (2 x 2 x 5 inches), one-piece metal box containing a sensor, software, and circuitry. The box aids in transmitting the vibrations of the fan to the sensor. When an abnormal vibration is detected, it orders a shutdown of the fan. The fan is protected by circuit breakers but they only trip after failure has occurred. If the mechanical interference is minor, the fan many continue to operate, generating smoke and odor from the blade tip, rotor hub, or motor windings. With the monitor, this situation would cause the unit to interrupt power to the fan. A "FAN INOP" message will be displayed in the cockpit and the monitor displays the reason for the shutdown for diagnostic purposes.

About 175 monitors have been in service for the past five years with a major U. S. airline and accumulated more than 700,000 operating hours. With no monitor, the fan removal rate was 3.3 of 200 units in service and 23 percent of those removed were found to be fault-free or false alarms. With the monitors, the removal rate was 2.8 and all of those were found to have some sort of problem. None, however failed or produced smoke in flight.

The virtue of the system is more than just improved reliability of the A/C system, however. With the monitor, if there is smoke, its importance is heightened because the crew knows that it is not caused by a problem with a fan. They can look to more sinister problems with the electrical system.

NOTE: This news was provided by AM sister publication Air Safety Week. For more information about ASW visit www.aviationtoday.com.