Plane & Pilot
Tuesday, May 10, 2011

How Tight Is Tight?


Even if you’re really thorough, you can’t always tell during preflight whether something is coming apart



FORCED LANDING. A Cessna 172, like the one above, had to make a forced landing after a loss of power due to oil starvation. Maintenance had failed to tighten bolts for a new vacuum pump.
Look through a Piper Cherokee service manual from the 1970s, and you'll note that the manufacturer gets pretty specific about certain things, such as requiring spark plugs to be torqued into the cylinders at between 360 and 420 inch-pounds, and fuel primer jets to be tightened at 60 inch-pounds. Forces necessary to ensure holding power are frequently specified by manufacturers, but when they're not, mechanics are required to exercise sound judgment based on skill and experience. As pilots, this is just one more area in which we have to trust that our mechanics aren't going to let us down. The chances of a pilot being able to detect an improperly torqued part even in what seems to be a thorough preflight inspection are pretty slim. If installed properly, the spark plugs, fuel primer jets and numerous other parts on which your life depends will stay put despite the vibration, heat, cold and stresses that are part of everyday flying. When things aren't secured as they're supposed to be, it sometimes becomes the NTSB's business.

Beech 95-B55
In January, the NTSB finished its investigation of an accident that occurred on November 24, 2008. It was late morning when a twin-engine Beech 95-B55 crashed in a wooded area behind a house in Whites Creek, Tenn. The airplane had been on an IFR flight plan, and on the ground, a witness said there were low clouds and drizzle at the time of the accident. The airplane was on a business flight from Hot Springs, Ark., to Nashville International Airport (BNA), Nashville, Tenn. The commercial pilot and both of his passengers were killed.

Before takeoff, the pilot contacted the Memphis Air Route Traffic Control Center, and requested IFR clearance to the destination airport. The pilot was instructed that the flight was cleared as filed, to climb and maintain 5,000 feet, expect 7,000 feet 10 minutes after departure, given a transponder code of 5524, and advised of the clearance void time. The pilot correctly read back the clearance, and at 9:11 a.m., he established contact with the facility and advised the controller that the flight was climbing through 2,500 feet. The flight was radar identified and instructed to climb to 7,000 feet continuing toward the destination airport.

About 21 minutes before the accident, the pilot was instructed by Memphis Center to descend and maintain 4,000 feet. A few minutes later, the flight was handed off to Nashville Approach. The controller told the pilot that ATIS Sierra was current, and to expect vectors for an ILS approach to runway 20R. About three minutes later, at 10:36:24, the pilot was instructed to fly a heading of 055 degrees for sequencing. Radar showed that from 10:37:21 to 10:42:21, the airplane flew approximately the heading given by the controller. Then, it turned left of course. At 10:42:58, the controller alerted the pilot to another airplane and the pilot radioed, "Yeah, well, I'm all screwed up up here, so stick with me."

The controller asked the pilot if he needed help. The pilot asked how far he was from the localizer. The controller told him he was 12 miles from the localizer, and asked the pilot to make sure he wasn't confusing the John Tune Airport ILS frequency (110.3 MHz) with the BNA runway 20R ILS frequency of 111.3 MHz. The pilot replied with words to the effect that he had 111.3 selected.

The controller told the pilot to fly his present heading, and to expect a base and final turn to the localizer. The pilot didn't respond. The controller again advised the pilot to expect a base turn in five miles, and a turn south onto the localizer. The pilot radioed, "Gotcha." At 10:44:24, the controller called the pilot and he replied, "I've got you, but I'm having trouble hold...." There was background noise followed by grunting sounds. The controller asked the pilot if he needed help, and he responded, "I got it into a spin and I can't stop it." Heavy breathing and more grunting sounds followed. At 10:44:54, the controller advised the pilot to climb immediately to 3,000 feet. The pilot responded with, "Got no climb." At 10:45:09, the controller again told the pilot to climb immediately. There was no further response from the pilot. The controller then broadcast in the blind that the airplane was six miles north of John Tune Airport, and that rescue vehicles had been dispatched.

Radar data indicated that the ground speed of the airplane dropped to only 51 knots when it was less than one-half mile from the accident site, a sure sign that the pilot was having difficulty controlling airspeed.

A witness saw the airplane flying in a southwesterly direction. The witness told investigators the airplane began spinning in a counterclockwise direction, and that it was flat while spinning. The witness estimated the airplane spun four times before it went out of sight. Each rotation took approximately one second. He called 911, and ran to the scene. When he arrived there, he noticed a fire at the front of the airplane that spread out. He estimated the fire department arrived in five minutes.

The pilot, age 67, held a commercial pilot certificate with ratings for airplane single-engine land, airplane multi-engine land and instrument airplane. His logbook wasn't located, but investigators estimated from other records that he had about 2,000 total flight hours.

Investigators determined that the left engine wasn't operating when the airplane crashed. About eight months before the accident, both propellers and propeller governors were removed for overhaul, and both engines were removed and replaced with remanufactured engines. Logbooks revealed that about six months before the accident, the mechanic who installed the engines changed the engine oil and cleaned the oil screens of both engines. The entry for the left engine indicated, "…Ground Run. No Leaks Noted At This Time." There was no further entry in the left-engine logbook following the oil change indicating any further maintenance was performed.

Examination of the left propeller revealed that it had not been feathered following the loss of engine power. Investigators found the fuel-supply line from the firewall to the left engine's fuel pump disconnected from the fuel-pump inlet fitting. They concluded that the disconnect wasn't a result of impact damage. During operational testing of the left engine, the B-nut at the engine-driven fuel-pump-supply inlet fitting was made only finger tight, not torqued to the value specified by the airframe manufacturer. Approximately two minutes into the engine run, the fuel hose separated from the inlet fitting. During the loosening process, fuel leakage was noted, but the engine continued to operate at the rpm setting investigators selected until the hose completely separated. Examination of the fitting by the Safety Boards' Materials Laboratory in Washington, D.C., confirmed that the fitting had been loose for some time before finally separating during the accident flight and causing the total loss of left-engine power.

The NTSB determined that the probable cause of this accident was the pilot failing to feather the left propeller and secure the left engine following the power loss, and his subsequent failure to maintain airspeed and lateral and directional control of the airplane. Contributing to the accident was the failure of maintenance personnel to properly tighten the fuel-supply hose at the engine-driven fuel pump.

Cessna 172M
On June 30, 2008, a Cessna 172M had to make a forced landing near Oakland, Calif. The commercial pilot and his passenger received minor injuries. Weather was good for the traffic observation flight. The pilot said that when the engine began surging and running rough, he adjusted the mixture to full-rich position, which seemed to help. Seconds later, the pilot noticed that the engine oil-temperature gauge was rising. He elected to divert toward the Oakland International Airport. Before they could reach the airport, the engine lost power. He headed for an open lot. During the landing roll, the airplane hit a large mound of dirt.

The bottom of the airplane was coated with oil from the engine to the tail. The engine dipstick indicated no oil was in the sump. All four nuts that secured the vacuum pump to the mounting pad were found loose. Oil was observed on the rear accessory housing below the vacuum pump and between the left and right magnetos. When compressed air was put into the oil cooler inlet line, bubbling oil came out of the vacuum-pump mounting pad.

Investigators found that the vacuum pump was replaced after the flight that took place before the accident flight. The FAA inspector who responded to the accident site interviewed the mechanic who put in the vacuum pump. The mechanic said he tightened all the nuts holding the vacuum pump except for the bottom left, which he couldn't reach. According to the FAA, the mechanic said he used a screwdriver and hammer to tighten that nut.

The NTSB determined that the probable cause of this accident was a loss of engine power due to oil starvation. The oil-starvation event was due to the failure of maintenance personnel to tighten the mounting bolts for the newly installed vacuum pump.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.



1 Comment

Add Comment