Mar 25, 2007


Due to system redundancy, it is permissible to operate an airliner with some items not working. The Minimum Equipment List (MEL) designates when and how this can be accomplished. Engineers try to envisage all the possible ramifications caused by the un-serviceable component. Sometimes they don't think of everything. For example the following accident report says:

"... At the time, a battery powered back-up source for instruments was not required on commercial aircraft...."

I suspect that a battery back-up source for the flight instruments was thought to be unnecessary because the 727 has three engine-driven generators. How could all three fail at once? Unfortunately, the B727's normal electrical loads exceed the capacity of two generators, let alone one. The Second Officer's most important duty in the event of generator problems is to protect the source of power to the Essential Bus which feeds the crucial flight instruments. He than must immediately reduce electrical loads by turning off a specific array of high-load items.

In this accident, the flight began with a handicap. When the last remaining generator went off-line, the pilots lost control of the aircraft before the Flight Engineer could re-establish power.

United Airlines Flight 266
Aircraft type Boeing 727-22C
Operator United Airlines
Tail number N7434U
Passengers 32
Crew 6
Survivors 0

United Airlines Flight 266 was a scheduled flight from Los Angeles International Airport, Los Angeles, California to General Mitchell International Airport, Milwaukee, Wisconsin via Stapleton International Airport, Denver, Colorado with 38 on board. On January 18, 1969 at approximately 18:21 PST it crashed into Santa Monica Bay, Pacific Ocean approximately 11.5 miles west of Los Angeles International Airport four minutes after takeoff.

Two minutes into its flight, the pilots reported a fire warning in the No. 1 engine and shut it down. The aircraft had departed LAX with one of its three generators inoperable, and shutting down the suspect engine took a second generator offline. The remaining generator became overloaded and shut down, resulting in the loss of all electrical power.

The pilots began flying in total darkness with less than 3 miles visibility due to fog and rain, with no lights or instruments, and consequently lost complete control of the aircraft due to disorientation and crashed killing all 38.

At the time, a battery powered back-up source for instruments was not required on commercial aircraft. The accident prompted the Federal Aviation Administration to require all transport category aircraft to have new backup instrumentation installed, and powered by a source independent of the generators.

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Now for something related but a little more light-hearted:

Captain Mann and the crew of the Indestructible 2 finally reach the safety of hyper-space.


Anonymous said...

What was the FAA thinking? Three generators won't do one bit of good in the event of fuel exhaustion or triple flameout. These are not supposed to be 100% non-survivable incidents

Aluwings said...

This website had the following elaboration about this accident:

Only captain and first officer primary flight instruments, but no standby instruments, were installed on early Boeing commercial airplanes (fig. 1). The two artificial horizons (attitude indicators) are powered by analog signals from remotely located vertical gyros. Both gyros are lost if all the main airplane generators fail. Because of this, the U.S. Federal Aviation Administration required a third, panel-mounted gyro instrument to be installed in the center of the instrument panel on later models. The third gyro is ac powered by a static inverter that receives its dc power from the main airplane battery.

The airspeed indicators and altimeters directly receive pitot and static pressure information from left and right pitot probes and static pressure from two pairs of flush-mounted static ports. These indicators require electrical power only for lighting.

If I understand this, it seems that the pilots lost the artificial horizons but had "raw air data" - that is airspeed and altitude information. But when all the power went out they lost the lighting to see these instruments.

moe darbandi said...

I remember reading a black box book with a bunch of accidents transcripts via the CVRs, and I believe this incident was one of the ones.

I'm not entirely sure which book it was, as it was a while ago, but I think it was this one

Aluwings said...

Thanks Moe, it may not be a "lulu" but it looks like a doozy. I just read the sample pages and already find it gripping. This is now on my reading wish list.