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Thursday, June 1, 2006

Avionics Etc.

Running the Relay Obstacle Race

Flightcrews Dealing With the Electrically Bizarre

By John Sampson

Some aircraft systems are electrically actuated, a few are pneumatic, but most flight-controls and ancillary services are hydraulically moved. However, the common denominator is DC electrical selection. So when the non-wiggly amps start moving erratically, the whole house of cards falls down. Thus it was on July 20, 1997, on an Aer Lingus B737-500 on approach to Copenhagen. After making a speedbrake selection the crew lost a myriad of indications, services and systems. Gear indications were lost and gear remained down on go-round. Some flight instruments failed partially (EADI, the Electronic Attitude Director Indicator, loss of color). There was no N1 engine RPM, Fuel Flow or Standby Horizon. There was additionally a warning for speed limit, a light in SPEED TRIM FAIL, MACH TRIM FAIL, AUTO SLAT FAIL and bell and cabin address systems were not working.

There was no single checklist that covered all the faults noted by the crew, so they elected to complete the Gear Does Not Retract checklist. That achieved nothing and the crew began to suspect a computer fault. Declaring an emergency, the crew commenced an instrument approach (ILS) and began disconnecting generators to check for an electrical transfer hiccup. There was no change, but later the symptoms suddenly disappeared and the aircraft landed normally. On taxi-in, as the pilot started a Public Address (PA) announcement, the announcement cut off, systems failed and smoke started filling the cabin. An evacuation was ordered (i.e., passed orally due to the PA failure).

The B737 DC power design has 56 essential systems coming off the battery bus. This bus is normally powered by transformer-rectifier #3, and alternately, the battery via the Battery Bus Relay R1. The welding of R1's contacts caused electrical arcing, which melted the relay's contact arm. The only indication of a supply fault was the 56 systems becoming currentless. This scenario isn't covered in any flightcrew manual. However it is possible to reestablish a current supply by selecting the STANDBY POWER switch to BAT - but only if the pilots are aware of that being required. Boeing tested a number of B737 relays and found similar wear patterns. B737 airplanes with an extended standby power configuration showed an in-rush current of about 114 amps and a steady state current of around 20 amps. However, in airplanes where the standard standby system powers the static inverter from the battery bus, the in-rush current is closer to 500 amps.

The National Transportation Safety Board (NTSB) turned up two prior cases of in-service battery-bus relay failure in the U.S. - both involving dense smoke. At that stage, the Boeing database contained eight failures since 1990, three of those since 1994. The solution was a relay with a better inrush tolerance. The fix was assumed to be "in."

Unsurprisingly, when the scenario replayed yet again on March 22, 2005, the UK Air Accidents Investigation Branch (AAIB) was scathingly critical of the FAA and Boeing in its just released report. When that incident occurred to EasyJet's B737-300, there was still no flightcrew procedure. The Easy crew was nonplussed but coped well. With this failure there is a risk that, due to the loss of power to the equipment cooling fans, all attitude information could eventually be lost if power is not switched to an alternate supply. That could be catastrophic.

Boeing published a Flight Operations Technical Bulletin, saying: Boeing has no technical objection to an airline incorporating a loss of Battery Bus procedure in their Operations Manual. However, since there are so many different electrical configurations throughout the 737 fleet, Boeing is unable to publish a generic procedure in the Boeing Operations Manual, which will work for all 737-300/400/500 airplanes.

The question then becomes whether the relay should be regularly inspected for wear and/or arbitrarily changed, based upon cycles. How many nasty perplexing incidents are required to kickstart an avoidance measure? Dense smoke overlaid upon obscure cascading electrical failures is a recipe for disaster.

John Sampson is the technical director for the International Aviation Safety Association, IASA. He has been involved in aircraft technical issues, including avionics, for more than 30 years.

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