Helvetic F100 at Nuremberg on Jan 20th 2015, uncontained APU failure

Last Update: February 12, 2018 / 16:36:23 GMT/Zulu time

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Incident Facts

Date of incident
Jan 20, 2015

Classification
Accident

Airline
Helvetic

Flight number
LX-1189

Aircraft Registration
HB-JVE

Aircraft Type
Fokker 100

ICAO Type Designator
F100

A Helvetic Airlines Fokker 100 on behalf of Swiss, registration HB-JVE performing flight LX-1189 from Nuremberg (Germany) to Zurich (Switzerland), was just being de-iced in preparation for departure from Nuremberg when the APU emitted a loud bang, a maintenance access panel jumped open releasing streaks of flames followed by a second loud bang and a pressure wave that shook the de-icing vehicle and had the de-icer duck for cover in his basket. Debris from the APU struck through the aft walls of the cabin and hit the backs of flight attendant seats. The crew received indication of an APU malfunction, the APU was shut down automatically.

The BFU reported in their monthly January bulletin released on Jun 15th 2015, that the aircraft received substantial damage including damage to the pressure vessel of the aircraft. There were no injuries. The occurrence was rated an accident and is being investigated.

The BFU reported that a post occurrence examination showed the APU cover had been ripped wide open and opened direct view to the APU core. The compressor disk had fractured, the turbine disk was seriously damaged. Pieces of the APU had penetrated the pressure vessel and the aft cabin also hitting and penetrating a flight attendant's seat coming to a stop in the container for the flight attendant's oxygen mask, where the debris melted into.

The occurrence aircraft returned to service on March 7th 2015.

On Feb 12th 2018 the BFU released their final report concluding the probable cause of the accident was:

The rupture of the APU and the resulting damage was caused by the ingestion and subsequent ignition of de-icing fluid. Additional factors were the limited field of vision and the insufficient marking.

The BFU stated following "Factual information":

The Auxiliary Power Unit (APU) ingested de-icing fluid during the de-icing procedure on the apron. Subsequently turbine speed increased strongly and the APU ruptured. As a result the aft pressure bulkhead of the airplane was punctured by debris.

The BFU reported the de-icer was just applying de-icing fluid type I to the leading edge of the right hand side of the vertical stabilizer, when the rotor speed (rpms) of the APU increased strongly and suddenly, the noise from the APU getting louder and the frequency increasing, the exhaust fumes increased strongly. The de-icer immediately aborted the de-icing and closed the jet tube. A loud bang occurred, the de-icer sought cover within his de-icing basket. More intense bangs occurred and the APU was shut off. During these events the maintenance door in the fuselage bottom sprung open and released a about 2m long flame. Blast waves of two bangs were so violent that the whole de-icing truck rocked.

The crew heard both bangs, too, and felt the aircraft was rocking. An APU error message illuminated, they shut the APU down.

A piece of the fractured APU penetrated the aft pressure bulkhead, smoke was emitted for a short time.

The BFU analysed:

Because de-icing fluid was ingested through the APU air intake, the rotational speed of the turbine began to increase. Once the threshold value was reached, the APU shut down automatically, and the fuel supply was cut off. Since the de-icing fluid had ignited and caused the increase of rotational speed, the shut-off fuel supply had no effect. The rotational speed increased further until the APU ruptured.

Subsequently, parts of the compressor wheel penetrated the aft pressure bulkhead in the area beneath the flight attendants' seats. The hot fracture piece was found embedded in the container for the smoke hoods. Smoke in the aft part of the cabin was the result. Passengers and cabin crew were prompted to move to the front part of the airplane. During a similar occurrence in the past almost the same area of the pressure bulkhead was penetrated. The BFU is of the opinion that in case the APU ruptures for other reasons it cannot be ruled out that the same area of the pressure bulkhead would be penetrated again.

The location of the APU air intake, on the right-hand side next to the fuselage and vertical tail fairing, poses the risk of de-icing fluid entering the open APU air intake during operation. The work of the de-icing personnel is made more difficult by the fact that due to the differences in temperatures fog develops which restricts the field of vision of the de-icing personnel. The APU air intake is not marked as other areas are, such as static ports. Europe has not implemented the safety recommendation the NTSB had issued due to a similar case requiring shutting down the APU during deicing.

The manufacturer had described the disadvantages extensively and these were the reasons why the safety recommendation was not implemented. This case shows, however, that the area of the air intake should be clearly marked to remind the de-icing personnel of the hazards. Other manufacturers, where the APU air intake location is comparable to the ones described here, set an example.
Aircraft Registration Data
Registration mark
HB-JVE
Country of Registration
Switzerland
Date of Registration
Certification Basis
Airworthyness Category
Nmcgdjhcfm cgnl Subscribe to unlock
Legal Basis
TCDS Ident. No.
Manufacturer
FOKKER AIRCRAFT B.V.
Aircraft Model / Type
F28 MARK 0100
ICAO Aircraft Type
F100
Year of Manufacture
Serial Number
Aircraft Address / Mode S Code (HEX)
Max. Operational Passenger Seating Capacity (MOPSC), indicative
Minimum Crew
Maximum Take off Mass (MTOM) [kg]
Engine Count
Engine
Incident Facts

Date of incident
Jan 20, 2015

Classification
Accident

Airline
Helvetic

Flight number
LX-1189

Aircraft Registration
HB-JVE

Aircraft Type
Fokker 100

ICAO Type Designator
F100

This article is published under license from Avherald.com. © of text by Avherald.com.
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