ELTs Blowing Fuses

ELTs on new Cessnas and others may not work because of improper handling


The following information is derived from the FAAs Service Difficulty Reports and Aviation Maintenance Alerts. Click here to view “Airworthiness Directives.”


An FAA avionics inspector in the Scottsdale, Ariz., FSDO reported that he has found the ELT remote switch circuits to be defective on two separate new Cessna 172s.

The Pointer 3000-11 model ELTs were both factory installed as original equipment by Cessna. An investigation by the manufacturer determined that the internal fuse had failed, but the manufacturer claimed the problem had been traced to Cessnas post-production test procedure several years ago.

The problem was fixed by upgrading the fuse from amp to 1 amp, and Cessna changed its test procedure. However, neither Cessna nor Pointer have attempted to upgrade existing models to the higher amperage fuse.

Maintenance personnel upgraded one of the failed units to the larger fuse, but concluded that the fuse apparently is not an adequate fix. Cessnas engineering staff confirmed the conclusion.

Improper handling during testing can result in ELT battery voltage being induced into the remote switch electronic circuit, which would cause even the 1 amp fuse to fail.

Owners and maintenance personnel should know that that these ELTs may not activate properly in an emergency. Aircraft with the Model 3000-11 ELTs installed should be inspected to ensure the proper function of the remote activation switch circuit.

Beech K35 Bonanza
Alternator Failure

During flight, the alternator failed, followed by loss of battery power. The pilot made a safe emergency landing.Maintenance personnel discovered the lower alternator attachment bolt pulled out of the bracket. This caused the alternator drive belt to lose tension. Only two or three threads engaged the lower attachment bolt, which failed to secure the alternator during operation. Previous maintenance personnel had installed several washers under the bolthead, which prevented proper engagement of the threads in the alternator bracket.

Beech V35A Bonanza
Alternator Failure

After installing a new alternator, the unit failed during an operational test.The alternator was removed, and an inspection revealed the failure was caused by lack of contact between the brushes and the commutator. When the alternator was assembled, an epoxy-type compound was used to hold the brushes in the brush holders. This kept the brushes from making contact with the commutator.Part total time – 0 hours.

Beech A36 Bonanza
Wing Flap Component Failure

During a landing approach, the 14-degree wing flap limit switch failed. The pilot made a safe landing.When the switch failed, the motor became overworked and the circuitry and wiring was burned on the power control board relay assembly. Maintenance replaced the flap motor and the control board. During an operational test, the motor malfunctioned fried the new PCB without opening the flap motor circuit breaker.Part total time – 506 hours.

Beech 58TC Baron
Misrouted Rudder Cable

During an annual inspection, the technician discovered the rudder cable was misrouted.The rudder cable was routed across a bulkhead instead of through it. The cable tension was checked and was far below the specified tension. There was paint over-spray on the cable and terminals, which indicates this condition had existed for a long time.Part total time – 3,122 hours.

Cessna Model 172S Skyhawk
Poor Quality Control

During the ferry flight from the factory, a new Cessna 172 experienced fluctuation of the altimeter and VSI. An inspection revealed the static line attachment to the static line selector valve was cross-threaded and leaking. The parts were replaced and the problem was solved. During the same inspection, however, the technician also discovered the trim wheel was incorrectly adjusted. Part total time – 9 hours.

Cessna 182S Skylane
Blown Exhaust Gasket

The pilot reported exhaust fumes in the cabin. The technician found a blown out number one cylinder exhaust gasket and discovered attachment hardware was missing. The right heat shroud slid down to the exhaust muffler and allowed fumes to enter the cabin through the heater system.Part total time – 146 hours.

Cessna 550 Citation II
Faulty Flapper Valve

After the installation of a new flapper-type check valve, the technician pressure checked the aircraft with a test unit. An excessive cabin leak was traced back to failure of the check valve. The rubber liner began separating from the valve and folded over which prevented the part from closing off properly. The part was approximately one month old.

Piper PA31-325 Navajo
Stuck Microswitch Plunger

When the pilot attempted to lower the landing gear, the left main landing gear green light did not illuminate, and the warning horn sounded. The pilot recycled the landing gear several times before landing without incident.A technician inspected the aircraft and discovered that the left main landing gears spring loaded plunger on the microswitch stuck and did not extend to its full travel position. The technician cleaned and lubricated the plunger, as well as all other switches as a precautionary measure. Part total time – 6,539 hours.

Piper PA38-112 Tomahawk
Oil Leak

The aircraft was sent to maintenance due to an oil leak.The technician discovered oil leaking from the push rod of the number one cylinder. A pressure check revealed the leak originated under the intake push rod at the head/barrel joint in the cylinder. A 2-inch long crack developed about 1 inches from the top of the chromed cylinder wall.Part time since overhaul – 1,434 hours.

Piper PA46-310P Malibu
Vacuum Pump Pressure Manifold

The aircraft operator experienced several standby vacuum pump failures. The pumps operate continuously.During the last pump replacement the parts showed signs of severe overheating. Closer investigation indicates a defective pressure manifold valve, which allowed pressure from the primary vacuum pump to backup into the standby pump may have caused the problem. The manufacturers product reference manual recommends pressure manifold valve replacement after 10 years of service. This manifold valve was 12 years old.Part total time – 1,907 hours.

Extra 300S
Empennage Structural Failure

After finding structural anomalies on several aircraft, the manufacturer issued SB 300-2-95, to prevent structural cracking just forward of the forward horizontal stabilizer attachment point.A pilot reported the in-flight failure of the left longeron and a crossmember. The failure occurred during aerobatic flight, but the pilot made a safe landing. SB 300-2-95 requires welding of the structural components. The manufacturer speculated that excessive heat may cause hydrogen embrittlement in some cases.


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