Iran Aseman F100 at Ilaam and Tehran on Mar 22nd 2019, right main gear did not extend, then by luck extended one last time

Last Update: October 19, 2019 / 16:31:07 GMT/Zulu time

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Incident Facts

Date of incident
Mar 22, 2019

Classification
Incident

Aircraft Registration
EP-ATG

Aircraft Type
Fokker 100

ICAO Type Designator
F100

On Oct 18th 2019 Iran's CAO released their final report in Persian only (Editorial Note: To serve the purpose of global prevention of the repeat of causes leading to an occurrence an additional timely release of all occurrence reports in the only world spanning aviation language English would be necessary, a Persian only release does not achieve this purpose as set by ICAO annex 13 and just forces many aviators to waste much more time and effort each in trying to understand the circumstances leading to the occurrence. Aviators operating internationally are required to read/speak English besides their local language, investigators need to be able to read/write/speak English to communicate with their counterparts all around the globe; thanks a lot to a native Persian speaker who volunteered to help with the translation). The report concludes the probable causes of the serious incident were:

Factors causing the incident, in order of importance, are:

- Faulty design of the landing gear system
- Contamination of the hydraulic system due to negligence of Aseman maintenance personnel
- Insufficient attention to cleanness standards (Part 145) at Aseman maintenance department
- Failure to notice the fault in the system during the preceding flights

The CAO reported that the aircraft had flown 40,110 flight hours in 34,329 cycles before the accident. It had flown 50 hours in 40 cycles in the 12 days before the incident, during which time problems with the landing gear were reported several times. After persisting for 5 consecutive days, the problem was finally solved, only to resurface 5 days later on March 22nd. FDR records from previous flights also indicate that the left and right gears had extension time discrepancies that, if noticed by the crew, would have suggested a problem with the right gear restrictor check valve.

Following the occurrence the aircraft was checked in Aseman’s maintenance hangar where it was observed that the main gear could not be extended in either normal or manual mode. An inspection of the hydraulic system indicated that parts of an O-ring had cut through the screen filter, blocked the restrictor check valve and prevented pressure from being removed off the actuator.

The investigators inspected all lines and modules, from the actuator to the first filter in LG selector valve, in order to find the origin of the dislocated O-ring, but only managed to find other parts of the O-ring. On manufacturer’s request, the O-ring was sent to Netherlands for further inspection. The manufacturer reported that the origin of the O-ring could not be determined, however since the O-ring looked new it was possible that it had entered the system during a maintenance operation. The LG selector valve was also inspected at Aseman where the filter was confirmed to be intact, showing that nothing had entered the system through the reservoir.
It should be noted that the failure of the screen filter had been a longstanding problem on F100 aircraft, and had caused several incidents both in Iran and elsewhere. There had been several modifications to the system and technical directives by Fokker the latest of which happened after this incident when Fokker confirmed design problems, modified the RCV design and issued directives AD2019-0104 and SBF100-32-166R1.

The CAO analysed, that as part of a C Check in October 2018, parts of the system between the selector valve and the actuators had been removed for maintenance. During the procedure and due to negligence, an O-ring had entered the lines. The O-ring then disintegrated within the next few days and moved through the system. The debris was caught in the screen filter, devised to protect the actuator RCV from contamination. However, with time and because of extensive hydraulic pressure (3000 psi) the contamination was pushed into the filter openings, severing the screen and partially blocking the flow. This had caused the disparity between the left and right main gear extension times, but had gone unnoticed by the crew who were preoccupied reading approach and landing checklists. On the day of the incident, the contamination caused a total blocking of the RCV which prevented the removal of hydraulic fluid from behind the actuator. As a result, the gear failed to open.
Incident Facts

Date of incident
Mar 22, 2019

Classification
Incident

Aircraft Registration
EP-ATG

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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