The Case of the Faulty Flight Director

Several Logan International Airport (Boston) runways end at the water’s edge: Seawalls protect them from the ocean storms. In 1973, a smallish commercial jet airliner approached Logan under limited visibility conditions. The aircraft was coming in steep and fast and by the time the pilot appreciated the situation, the airplane was doomed. It clipped the top of a sea wall, flipped over onto its top and skidded down the runway, catching fire in the process. The sole survivor perished a bit later from injuries.

The Scene of the Crime

The airline attributed the crash to pilot error and proceeded to settle passenger claims on that basis. The accident thus fell under workman’s compensation, which precludes suing one’s employer. A suit by members of the flight crew claimed the crash was due to the malfunctioning of the so-called flight director switch, exacerbated by poor FAA controlling and altimeter errors. The claim was that these factors directed the unsuspecting crew into the seawall. The court thought otherwise, attributing the crash to pilot and co-pilot error.

The plaintiffs appealed. Appellate court found that the flight director had clearly failed, but that the plaintiffs did not effectively make their case. The case was bounced back to federal court in Boston. The original judge was disqualified on basis of prejudice and bias and the case went into the hopper for a new judge. I entered the case at this point in my usual role as consulting metallurgist.

Listen to John Dodge's interview with Kenneth Russell as he explains how metal defects in a DC-9's flight director switch was the suspected culprit in a horrific 1973 plane crash in Boston. Listen Now

The Investigation

The flight director switch has five “decks,” (for an image of the flight director switch, see below). The round piece (stator) stays put while the other piece moves. The switch could be rotated to sort out the various electrical signals and give readings on important landing data, such as pitch and bank.

My client alleged that “arcing and sparking” led to metal particles shorting between contacts. Then the signals that indicated (for example) pitch and bank were corrupted, depriving the pilot of vital information.

Stored electrical energy may be discharged when contacts open, as in many gasoline engine ignition systems. The resulting electrical arc may be intense enough to melt particles of metal, much as occurs in arc welding. These errant particles may deposit in places where they cause trouble, as in creating short circuits. The electrical engineer is charged with designing switch circuitry that minimizes such effects.

I was given the disassembled switch for strictly nondestructive study. My main tool was the Scanning Electron Microscope (SEM). The depth of focus of the SEM enables the detailed study of rough surfaces at magnifications unattainable in optical (light) microscopes. The SEM also had an attachment for chemical analysis of the region being studied.

It showed the rotor contacts to be a silver-cadmium alloy. This is a very popular alloy, in part because it resists welding of contacts. All five decks showed heavy pitting of the contacts, as well as welded-on foreign particles. The switch was in pretty bad shape.

	Flight director switch

The Smoking Gun

The switch passed through a number of hands before me, and any loose or lightly welded particles had been lost. But, late in the case I had the opportunity to disassemble the same model switch with a similar service history. I was amazed at what I saw inside: The contacts needed a shave! There were big whiskers of sparked-off silver alloy all over the place. Shorting was a certainty.

About this time the defendant, the switch manufacturer, hired new counsel. The new firm had no experience in airplane crashes, but was very good at settling cases. At that time the maximum award for wrongful death was only a quarter of a million dollars. It appeared the defendant decided the legal fees and bad publicity from fighting the case just weren’t worth it, and that settlement was in order. My client expected the defendant to make him show the strength of his case, then settle on the courthouse steps.

The case was assigned to a newly appointed federal judge who was known less as a jurist than as a facilitator of settlements. The case settled before trial and my substantial fee (for then) was paid without protest. From this I gather the settlement was close to the maximum award.

Listen to John Dodge's interview with Kenneth Russell as he explains how metal defects in a DC-9's flight director switch was the suspected culprit in a horrific 1973 plane crash in Boston. Listen Now