Tiny Culprit

Improperly mounted resistor starts chain of events that puts Baron in the woods


Aviation, its been said, is intolerant of carelessness or neglect. Most pilots are concerned about their safety, at least to some extent, and pay more than lip service to keeping their skills sharp and their airplane worthy.

As human beings, however, its impossible to know everything about the airplane, the pilot, the trip and the weather. Trust is an inherent part of flying. You trust your mechanic to have competently maintained the airplane. You trust the briefer to provide you with a reasonably accurate picture of the weather en route. You trust controllers to sequence you safely and FAA technicians to properly maintain nav aids.

While many people think of flying as you against the sky, in fact it takes a lot of people to get you up there and keep you up there. Sometimes those people make mistakes. Sometimes the pilot catches the error. Sometimes not.

A Beech Baron operated by the state of Wisconsin took off one fall morning to transport two state employees from Madison, the capital, to the tiny town of Siren, some 205 nm to the northwest. By any measure it was the kind of trip general aviation airplanes do so well. Siren is far removed from airline service, driving round trip would consume an entire workday, and the Baron would eat up the miles in a couple of hours.

The pilot, a 52-year-old ATP with nearly 15,000 hours, was a state employee, qualified to fly most of the states fleet of 30 airplanes. He had been flying for the state for more than 13 years and held instructor ratings in airplanes and helicopters. He also was qualified to fly one of the states King Airs under Part 135. He had 1,200 hours in the Baron and had completed a proficiency check in the airplane a year earlier.

Just before 7 a.m. the pilot received his IFR clearance for the flight. There was a scattered layer at 8,000 feet but no ceiling. Although temperature and dewpoint were within a degree of each other, visibility was seven miles.

The flight departed Dane County Regional Airport just after 7 a.m. and climbed uneventfully to 6,000 feet. At 7:37, they checked in with Minneapolis Center at 8,000. By this point he was nearing Eau Claire and was given an altimeter setting. At 7:39, the pilot calmly transmitted, Were gonna return, were gonna go into, ah, Volk. We got some smoke coming out of this thing.

At this point, the airplane was 11.5 miles north of Bloyer Field in Tomah, 16.5 miles from Fort McCoy near Sparta and 16.8 miles from Volk, a military airfield at Camp Douglas.

Without hesitation, the controller replied, OK, youre cleared to Volk Field, ah, left or right turn, your discretion. Im showing Volk six oclock and, uh, 15 miles right now.

OK, and I guess you could declare it as an emergency. Were not gonna fool around. We got some sm[oke].

74Q, descend at pilots discretion then and maintain 6,000. Is it coming out of an engine?

No, its coming out of a heater, it seems.

Descend at pilots discretion. Maintain 4,000, left or right turn, your discretion. Reverse course, direct Volk when youre able.

Were gonna go ahead and head for Volk. We should be pretty much headed right at it now, I think.

During the course of this interchange, the airplane descended from a transponder-reported 8,100 feet to about 6,500 feet. Over the next two minutes it descended to about 3,800, about 1,900 feet agl. The pilot was told to switch frequencies to Chicago Center. Twenty seconds later, Chicago Center called the Baron. The pilot replied, 74Q, go ahead, bud. The plane was at 3,000 feet.

No further communications were received from the airplane.

Witnesses about a mile and a half from the crash site reported seeing the airplane flying southwest about 600 feet agl trailing dark smoke from the fuselage. The landing gear was up and two witnesses said it appeared the aircraft was under control.

The pilot apparently had a change of heart about his destination and was making a beeline for Bloyer Field, to the southwest, rather than Volk, to the southeast. He came up 2.2 miles short. There were no survivors.

The aircraft struck trees in what was estimated as a 20-degree descent and continued into a small clearing. There, it hit the ground violently and left a ground scar nearly 100 feet long. The airplane was shredded by the collision.

Smoking Gun
Investigators set out to find a cause for the crash. The radar unit, the aircraft heater and items reported to have been carried in the nose baggage compartment showed no signs of heat stress or smoke damage. They moved the investigation to the airplanes interior.

Burns in the carpet and right cabin sidewall, and smoke residue on the windows and instruments revealed a likely culprit. A resistor was found bolted to the right cabin sidewall near the forward bulkhead. Insulation was burned away around the resistor and four inches aft. The paint on the outside of the plane at that spot was heat-bubbled. Examination of the resistor showed it was covered with burned insulation residue.

The resistor was a component of an inflatable door seal, added under an STC. A second resistor and the air pump were not located at the crash site.

Another Baron operated by the same state agency also had the same inflatable door seal installed. The system was located in the same spot as the charred area on the accident airplane, and the two resistors were mounted to the side panel. It was removed, and charred insulation was found underneath. The STC called for the resistors to be mounted to metal, which would act as a heat sink. They were not correctly mounted in this installation.

The unit was tested by running it continuously. After about 45 minutes, the pump was 220 degrees F and the resistors were about 750 degrees. The pump motor failed after 51 minutes, at which time the resistors had reached 760 degrees. The accident flight had crashed 52 minutes after the pilot started the engines and got his clearance.

In the accident airplane, the door and its seal had been noted twice in the aircrafts discrepancy log in the previous month. Once was for the seal leaking. Hoses were replaced. The second time was for the Door Ajar light coming on. A switch was adjusted.

Ironically, two days before the accident the NTSB had recommended to the FAA that it issue an AD requiring the units to be immediately disconnected from the electrical system. Six weeks after the accident, the AD was issued.

Express Elevator Down
Although the door seal system was blamed for the fire, another link in the accident chain was also discovered.

From the time the pilot reported smoke to the crash, the pilot maintained communications with air traffic control, suggesting that he either did not consider the problem that serious or did not understand the implications of an electrical fire. In the Barons POH, the emergency sequence for electrical smoke or fire includes turning the battery and alternator switches off. The fact that communications and transponder operations continued shows the pilot did not take that step.

In addition, the POH stipulates that an emergency descent be made with the gear down and the flaps at the 15-degree approach setting.

Using that configuration, a state of Wisconsin pilot generated a descent rate of up to 6,200 feet per minute in a simulator, resulting in an off-airport landing in two minutes and 15 seconds. The accident pilot reported the smoke five to five and a half minutes before the crash.

Accidents usually involve chains of events, without any one of the accident will not happen. This chain involved subtle things.

A flaw in the design of the door seals allowed the unit to operate continuously if the seal leaked. The design went through the STC process and was installed on many airplanes before the flaw was detected.

The error in installing the system by not mounting the resistors on metal that could act as a heat sink was probably the most obvious one, in retrospect. Proper installation, however, is no guarantee that a fire wouldnt occur given the nature of the insulating material and the design flaw in the unit. Service Difficulty Reports showed four occurrences of smoke coming from the same model pump as installed on Barons in the eight years before the accident. Its not likely that all were installed incorrectly.

The pilots reluctance to point the nose down and descend like his life depended on it was the final link. Without the drag from having the gear and flaps extended, perhaps he was afraid of exceeding the airplanes envelope. Maybe he thought it wasnt too serious and didnt want to panic his passengers with such an extreme attitude. Maybe he didnt know the procedure and was just winging it.

Pilots learn to trust themselves and others. Sometimes other people let you down, but in the final analysis, maybe youre letting yourself down.

-by Ken Ibold


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