Qeshm F100 at Mashad on Feb 16th 2018, landed without left main gear
Last Update: December 26, 2018 / 11:49:56 GMT/Zulu time
The airline reported the aircraft suffered a technical malfunction. There were no injuries.
On Jul 12th 2018 the Dutch DSB reported that the occurrence was rated an accident and is being investigated by Iran's Accident Investigation Board. During the approach to Mashad's runway 31R the crew did not receive indication the left main gear had extended and locked, the crew went around and entered a hold to work the related checklist procedures, this however was without success. The crew decided to land without the left main gear being extended. The aircraft came to a stop on left main wing, right main gear and nose gear. The crew evacuated the aircraft.
On Dec 24th 2018 Iran's CAO released their final report (editorial note: in English! Thanks a lot!) concluding the probable causes of the accident were:
Regarding analyses of the reports, The Aircraft Accident Investigation Board (AAIB) determined that the main cause of this accident was “entering foreign contamination to hydraulic system by “maintenance activities during C Check“ which caused damage on blockage hydraulic flow in restrictor check valve .
The screen filter near hydraulic restrictor check valve was damaged by reflected force of Pollution and sediment available in the hydraulic system between and caused blockage the nozzle downstream to down lock LH landing gear mechanism.
- Insufficient surveillance on maintenance operation of aircraft.
- Lack of enough experience for Qeshm Air maintenance personnel.
The CAO analysed the sequence of events:
On February 16, 2018, at 13:08 (UTC), Qeshm Airlines flight 1202, a Fokker 100, EP-FQF, took off from Tehran Mehrabad International Airport (OIII) to destination Mashhad Shahid Hasheminejad International Airport (OIMM) with 97 passengers and 7 crew members onboard. The aircraft continued with FL330 and prepared for landing in RWY 31R. No abnormal condition was observed before landing gear extension while approaching to destination airport.
At 14:07 UTC time, the aircraft was in approach phase and copilot extended landing gear to prepare for landing.
On selection of landing gear down, an unsafe alert appeared as left main gear was not down and locked. As this condition persisted in spite of recycling attempts, the captain, who was the pilot flying, initiated holding. It was initially attempted to recycle the gear several times which unfortunately did not solve problem however he could recycle it once time. He could use alternate L/G procedure after first unsuccessful recycling. Abnormal procedure in Aircraft Quick Reference Handbook (QRH) has guidance for pilot in L/G unsafe condition as:
On establishing holding pattern, the pilot started the Landing Gear Gravity Extension as “Alternate L/G procedure” as stated in the quick reference handbook (QRH) but it was not successful.
The pilot made a low pass over the airport, which led the controller in the tower and a witness in the TWY G to conclude that the gear was “hanging in the bay” and only partially deployed.
Several attempts were done by pilots to resolve the problem:
- Two high-load-factor maneuvers with jolting the airplane hard on both left and right sides to free or loosen the gear has no apparent effect.
- Manual extension of landing gears to open L/G as effect of its weigh
- Touch down on the right main landing gear to cause the left main gear to be shake for free extension.
The pilot followed the gear up unsafe landing based on QRH. The cabin crew was fully informed of what was going on, and the passengers were briefed about the condition of flight. After all efforts to lower the left main gear have been exhausted, the cabin crew was told to expect an emergency landing, to order the brace position upon touchdown and to evacuate the cabin after the aircraft stops on the runway.
Finally the aircraft had RWY excursion and stopped on the left hand side of RWY 31R and evacuation was made successfully without any injuries and the airplane was substantially damaged. The airport had disabled aircraft removal plan but the plan was not done at time of accident completely and it was tried to transfer aircraft with crane and belt by internal service provider in the airport which caused more damages on aircraft fuselage.
The AAIB/CAO analysed the failure mode:
Following wreckage removal and displacing the aircraft to the safe place, due to the nature of accident and abnormal displacement of hydraulic linkages on left hand landing gear; the up pressure and /return, hosing restrictor check valve was opened and there was found that the inlet screen filter 3000psi (gear up selection) on restrictor check valve was ruptured; therefore this component was dispatched to Tehran for further investigations.
Pursuing the investigation in order to determine normal operation of, "the restrictor check valve ASSY” on Right L/G and effectiveness of filter in the “T union” in the up line of the retraction actuator which are located in left /right main landing gear hydraulic support lines, the both items were examined and checked, there were found in normal condition.
In Tehran the investigation shown that the size of white sediment particles was tiny, but the quality of material were still unknown. The largest black debris was entrapped in the hydraulic return line (restrictor orifice) and blocked the passage way. The black debris probably entered to the hydraulic system while opening hydraulic line of landing gear.
During the investigation, the evidences proved that the main reason for filter rupture was accumulation of debris at extremity of filter entrance which blocked the orifice and caused back pressure, consequently has raptured the filter (3000psi), which caused foreign contamination entered into the restrictor.
The design configuration of internal section of restrictor is in such a manner that in 3000psi, (gear up selection) the hydraulic fluid flows from the 7 orifices simultaneously and causes landing gear retraction in fastest way, but in gear down selection, the main gear extension is delayed as a result of restrictor which is assisted by spring pressure and make slower movement of main gear at extension.
In the Retraction mode, the hydraulic fluid passage in restrictor is done via 7 wide orifices and foreign contamination and Hydraulic sediment can flow easily from them. In extension mode, the interval hydraulic flows from a small orifice in mid-section of restrictor and the foreign contamination blocked the hydraulic flow between restrictor and main gear actuator at the time of accident and consequently the left main landing gear failed to open completely. The attempts of pilot to recycle landing gears could not change the condition of restrictor and force out the foreign contamination from mid hole.
During jacking time of the accident aircraft the Left main gear was extended with no restriction finally as a result of internal hydraulic leakage which was happened after accident.
The CAO/AAIB analysed the source of the contamination:
The aircraft landing gears were changed twice during “C Check”. The return line of Landing gear actuator had not required to be opened during removal or installation of landing gear. The involved maintenance tasks of “C check” were focused during the investigation of the aircraft maintenance base (FARSCO. maintenance center) and found respected area (The filter screen check valve on the return line of the actuator) was opened based on engineering order to comply AD No; 2015-0077 in accordance with Fokker Services Service Bulletin SBF28-32-123 and one time inspection of a hydraulic hose in accordance with the instructions of Fokker Services CSB-32-026.
However there was not observation of any abnormalities in hydraulic line and screen filter, but probably the foreign pollution entered to the system while restrictor connector was opened while performing one time inspection.
The origin of hydraulic contamination or sediment could not be detected because there were little amount of contamination to do chemical analysis however the shape of most contamination was same as aviation media blast material. The manufacturer should have a database for same occurrences to detect mitigation of associated risk in hydraulic system.
The right hand main gear restrictor and T-UNION (last chance filter) were inspected between the two hydraulic systems without any abnormal condition, therefore it was concluded that the aircraft hydraulic No;1 was in a clean condition.
Then both landing gears were installed on the aircraft and movement of foreign pollution in the hose of hydraulic system between T union and restrictor check valve caused damage on filter screen just after 240 flight cycles from latest “C Check”.
A0653/18 - RWY 31R/ 13L CLSD DUE TO DISABLE ACFT. 16 FEB 15:53 2018 UNTIL 16 FEB 18:00 2018 ESTIMATED. CREATED: 16 FEB 16:02 2018
OIMM 161600Z 11012KT 6000 NSC 03/00 Q1030 A3042=
OIMM 161530Z 10012KT 8000 NSC 04/00 Q1030 A3042=
OIMM 161500Z 10012KT 8000 NSC 05/00 Q1029 A3041=
OIMM 161430Z 10012KT 8000 NSC 06/01 Q1029 A3040=
OIMM 161400Z 11010KT CAVOK 07/00 Q1029 A3039=
OIMM 161330Z 11008KT CAVOK 07/M00 Q1029 A3039=
OIMM 161300Z 11012KT CAVOK 08/00 Q1029 A3039=
This article is published under license from Avherald.com. © of text by Avherald.com.
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