Transport Accident and Incident Investigataion BureauTransport Accident and Incident Investigataion Bureau
 
 
 
 

FINAL REPORT Nr.2/2006

ON THE SERIOUS INCIDENT

TO FOKKER 27-50 AIRCRAFT, REGISTRATION  YL-BAW OF  S/C ,,AIR BALTIC CORPORATION’’  FLIGHT BT- 443

on 18 MAY 2006 AT HELSINKI

The Aircraft Accident and Incident Investigation Bureau of the Republic of Latvia is a governmental, independent of all aviation authorities, organization established by law to investigate and determine the cause or probable cause of accidents and serious incidents that occurred in the civil aviation, as well if necessary for enhancing flight safety incidents.

The sole purpose of such investigation is in accordance with Annex 13 of the Convention of Chicago, as well as the Directive 94/56/EC of 21 November 1994, establishing the fundamental principles governing the investigation of civil aviation accidents and incidents of the Council of the European Union, to prevent accidents and incidents and, if the Bureau finds it appropriate, to make safety recommendations. The purpose of an investigation conducted under the responsibility of the Aircraft Accident and Incident Investigation Bureau Republic of Latvia is not to apportion blame or liability.   

 Address:        

  58 Brivibas Str., Riga

 LV-1011, Latvia

Phone.               7288140       ,        

Fax.     7283339,        

E-mail:            aaiib@aaiib.gov.lv

 Director of Aircraft Accident and                                                       Ivars Alfreds Gaveika

Incident Investigation Bureau

 

 

 FINAL REPORT Nr.2/2006

ON THE SERIOUS INCIDENT

TO FOKKER 27-50 AIRCRAFT, REGISTRATION  YL-BAW OF  S/C ,,AIR BALTIC CORPORATION’’  FLIGHT BT- 443

on 18 MAY 2006 AT HELSINKI

 

 

 

CONTENTS

 Synopsis

 General information

 Investigation

 1. FACTUAL INFORMATION 

 1.1. History of the flight

1.2. Injuries to persons

1.3. Damage to aircraft

1.4. Other damage

1.5. Personnel Information

1.6. Aircraft information

1.6.1. General

1.6.2. Hydraulic system description

1.6.3. Landing gear operation description

1.6.4. Flaps operation description

1.7. Meteorological information

1.8. Aids to Navigation

1.9. Communications

1.10. Aerodrome information

1.11. Flight recorders

1.12. Wreckage and impact information

1.13. Medical and pathological information

1.14. Fire

1.15. Survival aspects

1.16. Tests and research

1.17. Organizational and management information

1.18. Additional information

1.19. Useful or effective investigation techniques

 2. Analysis

 3. Conclusions

 4. Safety Recommendations

 Appendices:

 1. Radio communication records.     

2. Aircraft Flight Data transcription.

3. Flight Recording System Verification Check Sheet.

4. Latvian Environment, Geology and Meteorology Agency information about actual and

    forecast  an appreciation of the weather.  

5.  PIC and F/O Interviews.

6. Operational Flight plan.

7. Loadsheet of aircraft Fokker – 50 ,,YL-BAW”. 

8. ,,Air Baltic” Technical investigation report.

9. Aircraft PIC Airline Transport Pilot Licence.   

10.Aircraft PIC Proficiency check copy.

11. Aircraft PIC Medical Certificate Class I.

12.Aircraft F/O Commercial Pilot Licence.

13.Aircraft F/O Medical Certificate Class I.

14.A translation of the police report concerning the pilot's alcometer test.

 Abbreviations

 LO QTY  - Low Quantity

SG           - Symbol Generator

DFDR      -  Digital Flight Data Recorder

CVR        - Cockpit Voice Recorder

LH MLG - Left Hand Main Landing Gear

RH MLG - Right  Hand Main Landing Gear

PIC          - Pilot in Command

F/O           - First Officer

ATC         - Air Traffic Control

CPL         - Commercial Pilot Licence

ATPL       - Airline Transport Pilot Licence

CAP         - Central Annunciator Panel

IAU          - Integrated Alerting Unit

MCL        -  Master Caution Lights

EFIS         -  Electronic Flight Instrument System

UTC         -  Universal Time Coordinated

IAS           - Indicated Airspeed

TO            - Take-Off

MWL        - Master Warning Light

T E            - Trailing-Edge

AHRS       - Altitude and Heading Reference System

ADC         - Air Data Computer

FMS         -  Flight Management System

AFCS       -  Automatic Flight Control System

VOR         - VHF Omni Directional Range

ILS            - Instrument Landing System

DME         - Distance Measuring Equipment

ADF          -  Automatic Direction Finder

RA             -  Radio Altimeter

DU             -  Display Unit

C/A            -  Cabin Attendant

PSI             -  Pounds-force/sq. inch

FH              - Flight Hours

 Synopsis

 Unless stated otherwise all times in this Report are UTC time

 The incident to the Fokker 27-50  aircraft occurred on the first flight following a 3 day maintenance check. 10 minutes after take off  from Airport St. Petersburg (ULLI)  to Riga (EVRA) International Airport, during initial climb on May 18, 2006 having performed flight No BT- 443  aircraft and bird strike occurred, as well as hydraulic system problem, on the hydraulic panel LO QTY came on. During flight was indication that DFDR sometimes on, sometimes and SG 1 (left) failure.  Crew declared an emergency and landed at Helsinki airport Vantaa (EFHK). Passengers were informed of technical problem leading to diversion to alternate aerodrome.

Preliminary investigation revealed that, hydraulic hose in LH MLG is broken. DFDR and CVR were removed, crew interview   conducted by staff of Finland AIB.

 Inspection after bird strike was performed by “Air Baltic Corporation” staff.  Signs of bird strike were found on a windscreen, no damage found. Signs of bird strike were removed, DFDR and CVR were installed. During inspection in LH MLG bay was found that Retraction Actuator “UP” position pressure line flexible hose  was damaged. After replacing damaged and checking “UP” position pressure hose LH and RH engine-driven pumps were checked. As it is not possible to perform LG extension/retraction test, one technical ferry flight from HEL to RIX was permitted with Landing Gear down. In Riga Landing Gear extension/ retraction test was performed satisfactory and aircraft released to service with no limitation.

 

 General information of the serious incident

 Operator                                 -            S/C „Air Baltic Corporation”

 Aircraft Type                         -            F27 Mk 050

 Nationality                             -             Latvian Republic

 Registration                            -            YL-BAW

 Manufacturer                          -            Fokker Services B.V., the Netherlands

  Owner                                   -             FINOVA Aviation Leasing Ltd.;

 Date of manufacture               -            1989

 Place of Accident                   -             During initial climb after take off from  Airport

                                                               Pulkovo (ULLI), St Petersburg;          

 Date and time                         -              18 May 2006, approximately 10:32  

  Investigation

 The investigation was performed by investigators of the Aircraft Accidents & Incidents Investigation Bureau (AAIIB) Republic of Latvia. The Flight safety report No 060518 BT-443 on fax message form about ,,Air Baltic Corporation” airlines Fokker 27 Mk 050 aircraft YL- BAW Serious Aviation Incident was notified to the Aviation Accidents & Incidents Investigation Bureau (AAIB) from the AS “Air Baltic Corporation” on May 19,  2005 at 12:49 UTC.  

Thereto the notification on fax message form about “Air Baltic Corporation” airlines aircraft Fokker 27 Mk 050 YL- BAW Serious Aviation Incident was notified to the Aircraft Accidents & Incidents Investigation Bureau (AAIB) from the Air Accident Investigation Board (AIB), Helsinki, Finland   on May 23, 2006 at 17.01.UTC. The AIB Finland suggested that the responsibility of the investigation   would take AAIIB of the Republic of Latvia.

            In reply to proposal of Finland AIB the Aircraft Accidents & Incidents Investigation Bureau (AAIB) approved its responsibility to conduct investigation and all possible information and documentation concerning the incident, as well as broken parts removed from the aircraft by Air Baltic maintenance staff was requested. Delivery of documents, photographs and DFDR data (on CD-ROM), as well as damaged hydraulic hose were received from AIB Finland on June  29,  2006. Likewise all necessary information for conducting investigation from aircraft operator S/C “Air Baltic Corporation” was requested.  

   1.      Factual information

 1.1.          History of the flight

 The flight and the serious incident were reconstructed using crew statements, the digital flight data recorder (DFDR), cockpit voice recorder (CVR) and the air traffic control (ATC) transcript.

On May 18, 2006 the AIR BALTIC Fokker F27 Mk 050 with registration YL-BAW started the take off from St. Petersburg (ULLI) airport at 10:32   for a flight to ,,Riga” International Airport (EVRA), as flight No BT- 443. On board were 39 passengers, two cockpit crew - the Pilot in Command and the First Officer and two Cabin Attendants. The PIC occupied the left seat. Approximately 10 minutes after take off from St. Petersburg  during initial climb  aircraft and bird strike occurred. Crew continued a flight and  performing after take off normal check list, but they did not change configuration- they did not retract a flaps and did not select climb power yet. When a crew selected “Gear up ” was  problem with RH MLG. RH MLG was  unsafe after UP selection. According to procedure of abnormal checklist they must recycle, but when crew selected “Gear down” was  problem with LH MLG. RH MLG and NOSE GEAR were down and locked but  LH MLG  not extended after UP position. IAU produced “Double chime” and flashing “Master Caution Lights” on the CAP  as well on the hydraulic panel LO QTY and engine pumps Left and Right fault  came on. Crew selected “HYDRAULIC  DISPLAY SELECTOR” on position “QTY, %”, hydraulic display indicated hydraulic quantity was  9%. They discussed about possible cause of hydraulic system failure and suspected that it was hydraulic lines damage due to bird strike.  After that crew made calculation of fuel. Quantity of fuel will be enough for flight to Riga, but there was some problem with holding area and with aircraft configuration gear down limit of speed does not exceed 170kt and PIC made decision to flight alternate airdrome Helsinki  (EFHK).  When was reached FL180 crew alerted to St. Petersburg ATC an emergency, set transponder code 7700 and informed about hydraulic problem. After few minutes when they left Russia airspace they change frequency to Helsinki, declared ATC an emergency, announced about hydraulic problem and set transponder code 7700. Passengers were informed of technical problem leading to diversion to alternate aerodrome and expected time of arrival to Helsinki.

  Approaching to Helsinki airport crew made alternate down procedure for LG – selected “ALTERNATE LG SELECTOR” down and LH MLG position lights indicated that LH MLG is down and locked as well performed alternate procedure for flaps – selected ALTERNATE FLAP control switch that operates the flap drives electrically and after that landed in airport Helsinki  - Vantaa - Landing time –  11:46.  Before landing there was enough  pressure on indicator for using brakes but crew did not use brakes during landing because  it was not necessity.

Before departure from St. Petersburg  had not  indications of any problems of aircraft systems, hydraulic pressure but after departure  during the flight  was indication that DFDR sometimes on, sometimes of as well as SB 1 (left) failure.

  1.2. Injuries to persons

 None

 1.3. Damage to aircraft

  After performing inspection of bird strike signs on windscreen it was stated that no damage.

 1.4. Other damage

 Objects other than aircraft not damaged.

 1. 5. Personnel information

 Crew qualification according to determinated  requirements.

 PIC, male, age 41,  

Pilot qualification according to determinated requirements:

Licence                                               - ATPL(A) issued 12.01.1999., validity 05.03.2008.,

                                                            rating F- 50

Total flying experience                      -8000 hrs;

Flying experience as captain (PIC)    -1037 hrs;

Flying experience on aircraft type     -1100 hrs;

Flaying hours in incident day            - 07 hrs 20min;

Flaying time 72hrs before incident   - 20 hrs 30 min;

Flaying time 7days before incident   - 42hrs 48 min;

Last examination of pilot qualification - March 07, 2006;

Last medical examination                     - December, 2005.

Co-pilot                                               - male, age 30,  

Pilot qualification according to determinated requirements:

Licence                                                - CPL(A) issued 11.07.2005., validity 11.07.2006.,

                                                                 rating F- 50

Total flying experience                        -1100 hrs;

Flying experience on aircraft type       - 216 hrs 19 min;

Flaying hours in incident day              - 07 hrs 20min;

Flaying time 72 hrs before incident     - 22 hrs 17 min;

Flaying time 7days before incident      - 23 hrs 57 min;

 Last examination of pilot qualification - June 30, 2006;

Last medical examination                     -July, 2005.

 1.6. AIRCRAFT INFORMATION

 1.6.1. General

 Aircraft type – F.27 Mk 050;

Manufacturer-Fokker Services B.V., the Netherlands;

Manufacturer’s serial No – 20148;

Owner of aircraft - FINOVA Aviation Leasing Ltd.;

Registration  -  YL-BAW;

Validity of the Certificate of Registration –Date of issue March 15, 2006;

Year of manufacture – February 15, 1989;

Total aircraft flying hours- 27507  hrs;

Landings  - 41152 (on the May 18, 2006)

Flight hrs (since last periodic inspection) 3hrs (on the May 18, 2006 performed 3 days check);

Engines manufacturer and Model – Pratt & Whitney, Canada, PW 125B;

Serial No.:

Left – 124066;

Right – 124162;

The left (number1) engine had accumulated - 22703   hours;

The right (number2) engine had accumulated - 23401   hours;

Validity of the Certificate of Airworthiness – Valid until October 23, 2006.   

 1.6.2. Hydraulic system description

 The aircraft has a single hydraulic sys­tem for operation of the landing gear, nose-wheel steering, brakes, and flaps. The main components are a tank, two engine-driven pumps (one per engine), and an electrically driven pump. Addition­ally, fire shut-off valves are incorporated. The system is controlled from the HY­DRAULIC panel. Failures will be detected and the relevant alerts are presented.

Hydraulic power is supplied at a nominal system pressure of 3000 psi by the en­gine-driven pumps. One pump is suffi­cient to supply all hydraulic power re­quirements.

A low capacity electrically driven pump is installed. Its main purpose is for main­tenance system testing. It can be used to pressurize the brake system prior to en­gine starting.

            If a total hydraulic system failure occurs, normal operation of the landing gear, nose-wheel steering, brakes, and flaps is not possible.     

 1.6.3. Landing gear operation description

 The Landing Gear (LG) consists of a for­ward retracting nose gear and two rear­ward retracting main gears. Doors en­close the landing gear bays. The LG is retracted and extended hydraulically. In the event of a hydraulic system failure, the landing gear can be lowered by grav­ity.

The main gears are kept in retracted po­sition by hydraulic pressure. In case of pressure loss, the main gears are held in retracted position by mechanical up-locks. The main-gear doors open and close mechanically by the main-gear struts. When extended, the main gears will lock mechanically.

Hydraulic operation is controlled by a two position (UP or DOWN) LG selector, located at the center main instrument panel. The electrically controlled LG se­lector valve directs hydraulic pressure accordingly. On the ground UP selection is prevented by a selector lock. This lock will be released automatically as soon as the aircraft becomes airborne. In the event of an automatic lock-release failure, the lock can be released by de­pressing the LOCK OVERRIDE button.

            The ALTERNATE LG selector is located at the RH aft side of the pedestal, and has to be pulled upward to select alter­nate down. Operation of this lever re­leases the up-locks of main-gear and nose-gear doors, and dumps the hy­draulic pressure of landing gear and nose-wheel steering operation. Herewith the LG will extend by gravity and lock by spring force.

LG position lights are located at the LG selector panel. All lights are out when the gears are up and locked. The down-and-locked position of the main gears is indi­cated by one green light for each gear.  The second light is a backup locked down indication. If the main gear lights are inoperative, the locked down condition of each main gear may be checked by verifying, through the cabin windows, that the red lines on the lock strut are in line.

During retraction and extension a blue light, incorporated in the LG selector knob, comes on. When the light comes on due to an alternate down selection, it remains on until the LG selector is se­lected DOWN.

            Any disagreement between the position of the gears and the LG selector is indi­cated by amber lights at the LG selector panel. When the gears are in transit, no alert will be presented.

            When the LG is not down and locked, a level 3 alert is presented when flaps are selected 25 or more. The same level 3 alert will be presented when the LG is not down and locked and the rate of descent is in excess of 200 ft/min while below 830 ft radio altitude with a LH and/or RH POWER setting below TO. When the radio altitude is not available, an IAS below 140 kt will complete the above condition.

MWL and chime can be cancelled by se­lecting the LG DOWN or alternate down. The red LG light at the CAP goes out when all gears are down and locked. When any of the gears does not lock, the relevant level 2 alert will be presented.

 When the alerts are presented due to the rate of descent etc, MWL and chime can also be can­celled in the normal way. The LG light at the CAP goes out when the alert condition has been removed.

1.6.4. Flaps operation description

 The flaps can be controlled by a seven position FLAP selector (UP, 5,10,15, 20, 25, 35). The mechanically operated flap selector valve directs hydraulic pressure to the flap drive unit in accordance with FLAP selector position. The position of the flaps is indicated by a flap position in­dicator located at the center main instru­ment panel. Position markings are lo­cated at the inboard side of the engine nacelles.

A disagreement light comes on when flap position is not in accordance with FLAP selector position. The alert is inhibited during the period when the flaps are in transit.

 Alternate operation

Adjacent to the FLAP selector is an AL­TERNATE FLAP control switch that oper­ates the flap drive unit electrically. The travel range is the same as in the hy­draulic mode; however, the flaps extend at a slower rate. After alternate oper­ation, no hydraulic operation can be made before the system is reset. Reset is obtained on the ground by moving the FLAP selector to an other position.

1.6.5. Symbol Generator

 The aircraft is equipped with two independent Electronic Flight Instrument Sys­tems (EFIS), one on the PIC's side and one on the F/O's side. The purpose is to display the attitude and relative posi­tion of the aircraft. Each system consists of one Symbol Generator (SG), two Dis­play Units (DU) and one EFIS control panel. Additionally, other controls are  provided at the AVIONICS panel and at the Flight Mode Panel (FMP).

            Each SG receives data from AHRS, ADC, FMS, AFCS, VOR, ILS, DME, ADF, RA, marker beacon, and weather radar. It processes these data for presentation at the DU's. Input data are compared by the SG's; any difference detected will be displayed at the DU's Normally SG 1 drives the captain's DU's and SG 2 the first officer's DU's. In case of an SG failure the operative SG is capable to drive all four DU's. Alerts are presented in the event of an SG failure.

 1.6.6. Airworthiness and maintenance of the aircraft

 According to documentation given from S/C „Air Baltic Corporation” on May 17, 2006 aircraft weekly check was performed. During maintenance was found nose wheel No2 worn to unit. Was performed nose wheel No2 (RH) assembly replacement and testing.

On May 18, 2006  3 days check was performed. During 3 days check found strobe light on RH wing inoperative. Strobe light tube was replaced and tested.

FOKKER 50 3 days check inspection list Task Number & Reference 121305-BT-01, AMM  12-13-05-610-823, AMM 12-13-05-610-843 prescribe to carry out checking nose and main gear strut for extension and signs of hydraulic fluid leakage as well as wiping of strut pistons with a cloth soaked in hydraulic oil (MIL 5606). During 3 days checking were not found any deficiencies of aircraft hydraulic system.

According to information given by crew members during  interview any deficiencies of  aircraft before departure from airport Pulkovo  St. Petersburg performing Pre-flight inspection were not revealed.

1.7. Meteorological information

 Airport Pulkovo, St. Petersburg

 FCRS31 RUMS 180700

ULLI 180745Z 180918 16006MPS 9000 BKN020CB TEMPO 0918 2500 SHRA

BKN010CB=

 SARS31 RUMS 181000

ULLI 181000Z VRB03MPS CAVOK 15/M04 Q1017 NOSIG=

 SARS31 RUMS 181100

ULLI 181100Z VRB01MPS CAVOK 14/M03 Q1017 NOSIG=

 SARS31 RUMS 181200

ULLI 181200Z 17006MPS CAVOK 14/M04 Q1017 NOSIG RMK 17006G09MPS=

Airport Vantaa, Helsinki

 FCFI31 EFHK 180800 RRA

EFHK 180841Z 180918 17009KT CAVOK=

 SAFI31 EFHK 181020

METAR EFHK 181020Z 19010KT CAVOK 11/M04 Q1016 NOSIG=

SAFI31 EFHK 181050

METAR EFHK 181050Z 19007G17KT 130V230 CAVOK 12/M05 Q1016 NOSIG=

 SAFI31 EFHK 181120

METAR EFHK 181120Z 17006KT 120V220 CAVOK 12/M05 Q1015 NOSIG=

  1.8. Aids to Navigation

 Not applicable

  1.9. Communications

 The radio equipment functioned normally and had no relation with the cause of incident.

Phraseology of ATC  traffic controllers have conformed with „Procedures for Air Navigation Services - Air Traffic Management”- 14th Edition, 2001. (ICAO Doc. 4444)   Chapter 7 –Aerodrome and meteorological information, Items 7. 3. 1. 2. and 7.3.1.2.2.

  1.10.  Aerodrome information

 Airport Pulkovo st. Petersburg:

 Elevation  AMSL 24 m (79 ft), Direction 10R/28L Length 3,780, Surface Asphalt

                                                   Direction 10L/28R Length 3,397, Surface Asphalt

 Airport Vantaa Helsinki:

 Elevation  AMSL 55 m (179 ft), Direction 04R/22L, Length 3,440, Surface Asphalt

                                                    Direction 04L/22R,  Length 3,060, Surface Asphalt

                                                    Direction 15/33      Length   2,901, Surface Asphalt

   1.11. Flight recorders

 DFDR

The aircraft was equipped with a DFDR, Model UFDR, serial No 5688

The digital flight data recorder were processed at the Muirhead Avionics laboratory, Mallory House, Navigator Park, Middlesex UB2 5NH. United Kingdom. The Muirhead Avionics provided a factual report with the following information:

T E Flap position showed 8 degrees during flight until approach when full flaps selected.

Aircraft  Flight  recorders data transcription (see. Appendix  ).

 CVR

 The aircraft was equipped with a CVR produced by FAIRCHILD, model A100A, serial No 53119

 

Picture 1

Picture 2

 

1.12. Wreckage and impact information

 Not damage

 1.13. Medical and pathological information

 Not relevant to this incident

 1.14. Fire

 There was no fire

 1.15. Survival aspects

 Not necessity to survey

1.16. Tests and research

 Preliminary investigation revealed broken hydraulic “UP” pressure hose to Retraction Actuator cylinder in LH MLG bay. Hydraulic hose “Main Gear Up F8553-048-041  in LH MLG bay  (item 30 in attached illustration) was broken.

Visual  inspection shows that the braiding of the hose is damaged, and that the hose itself has hole in it. Damaged hydraulic flexible hose  with partnumber F8553-048-041  was sent for testing to National Aerospace Laboratory NLR, the Netherlands.

According to conclusion of National Aerospace Laboratory NLR the hose assembly failed because of a maintenance error. The damage to the braiding is consistent with a localized cyclic load in a torsional direction. The buckling load was present because the hose was mounted incorrectly (twisted). The curve of the hose should be in the plane of movement,

allowing  a gradually expanding motion when the landing gear moves. Each time the landing gear was cycled up or down under impact the buckling load reversed direction  eventually  bringing about hose failure by fatigue.

      

Picture 3 Overview of installation inside left main gear bay

 

 

 

 

 Picture 4

 1.17. Organizational and management information

 Not applicable

 1.18. Additional information

 Not applicable

 1.19. Useful or effective investigation techniques

 Not applicable

 2.    Analysis

 2.1. General

 The investigation of the serious incident to flight BT- 443 on May 18, 2006 was orientated essentially around the following questions:

-        Was the aircraft in an airworthy condition before departure from airport Pulkovo, St Petersburg?

-        Did bird strike influences have causative effect on the incident – hydraulic system failure?

-        Did Hydraulic hose “Main Gear Up F8553-048-041” have manufacturing defects?

-        Did maintenance procedures have an influence on the incident?

-        Flight crew actions.

2.2. Aircraft  airworthy condition

 Flight BT- 443 it was return flight to Riga  en route RigaSt. Petersburg - Riga  and flight from Riga to St. Petersburg was performed successfully. During landing  there were not any problems with hydraulic system, hydraulic pressure or landing gear. The cockpit crew members also declared, that aircraft was airworthy before departure from St. Petersburg.

          Analyzing these facts relevant to the case it is possible to a certain extent of probability to predicate that aircraft had  airworthy condition before departure from airport Pulkovo, St Petersburg.

 2.3. Bird strike influence

 

 

  Picture 6 (signs of bird strike)

 

Despite cockpit crew have founded suspicion, that hydraulic system failure occurred due to multiply bird strike and as a result of hydraulic lines may be had damage, because the hydraulic system failure occurred practically right away after bird strike, during inspection the LH MLG  compartment the bird strike signs had not found. Bird strike signs was found only on the RH windscreen as well as inspection showed, that accordingly to  the design and placement  hydraulic system elements  of  Fokker- 50 aircraft, hydraulic lines damage due to bird strike or other foreign body practically not possible.

2.3. Availability of manufacturing defects

 Such  a flexible hose failure on S/C „Air Baltic Corporation” Fokker- 50 occurred also on the RH MLG UP pressure line  on the October 10, 2005. As investigation shows, in both cases hydraulic leakage was from flexible hoses located in the UP line, where hydraulic pressure is 3000 PSI, after  first case aircraft operator suspects  these hoses have worn and hydraulic pressure breaks it due to wear and visual inspection was carried out on all airBaltic Fokker 50 fleet. According to Air Baltic Technical Air Baltic investigation report No 06-024 of June 06, 2006 information it was not found any visual damage on the   MLG hoses during that inspection.

After analyzing all information about probable reasons of damage and as it is impossible to check internal condition the replaced flexible hose with partnumber F8553-048-041 from Fokker- 50 YL-BAW  was sent for laboratory testing and determining  the damage mechanism and through that the cause of failure.

 2.4. Conduct of the maintenance

 The investigation found that the maintenance errors were not the result of willful negligence, or any desire to perform a less than satisfactory job, but the result of a combination of systemic issues, that had increased the probability of an error being committed. It is considered that installing the flexible hoses are standard job and there have not special technology for installation. Maintenance  personel had followed the accepted 'normal' practices in their work area and believed that they had acted correctly in doing so.

Hose's installing technology   was not developed and implemented.

Considering the fact that the failure was induced by fatigue, in effect by cycling the gear a large number of times, the thoroughness of the pre flight inspections can called in question.

A further contributory factor may be the lack of oversight and guidance by the licensed aircraft engineers responsible for the job performers. This may be due to the fact that they do not have any supervisory responsibilities and they therefore do not see it as their job to monitor the actions of the performers.

 2.5. Flight crew actions

 The flight crew made a positive decision alerted St. Petersburg ATC and Finland ATC an emergency, set transponder code 7700 and informed about hydraulic problem. PIC made decision to flight alternate airdrome Helsinki  (EFHK) because it have some runways. Had they not taken this action, the subsequent landing difficulties could have been further compounded, increasing the degree of risk.

 3.    Conclusions

 3.1. Findings

-        the aircraft was in an airworthy condition before departure from airport Pulkovo, St. Petersburg;

-        a bird strike  had not causative effect on the serious  incident – hydraulic system failure;

-        “UP” position hydraulic line(flexible hose) connected to Retraction Actuator of MLG in LH MLG bay was damaged, that lead to leakage and as a result of loss of the majority of the hydraulic fluid from reservoir;

-        visual inspection shows that the metallic reinforcement braiding of the hose is damaged, and that the hose itself has a hole in it;

-        damaged flexible hose have not manufacturing defects;

-        the damaged flexible hose was mounted on aircraft in manner that curve of the  hose was not in the plane of movement, the hose was mounted incorrectly (twisted);

-        aircraft operator's maintenance  view point  considered installing the flexible hoses are standard job and there have not special technology for installation;

-      the thoroughness of the pre-flight inspections can also be questioned;

-      the maintenance errors were not the result of willful negligence, or any desire to perform

     a less than satisfactory job, but the result of a combination of systemic issues, that had

     increased the probability of an error being committed;

-      after first case of the flexible hose failure hose's installing technology was not developed

  and implemented;

-      the cockpit crew was properly licensed to perform the flight;

-      the flight crew made a positive decision  alerting St. Petersburg ATC and Finland ATC an emergency  and to flight alternate airdrome Helsinki  (EFHK) because it have some runways for landing.

 3.2. Causal factors

 The following causal factors were identified:

 -        the damage to the hose braiding is consistent with a localized cyclic buckling load in a torsional direction;

-        the buckling load was present because the hose was mounted incorrectly (twisted). The curve of the hose should be in the plane of movement, allowing  a gradually expanding motion when the landing gear moves, each time the landing gear was cycled up or down the buckling load reversed direction, eventually bringing about failure by fatigue;

-        the hose assembly failed due to a maintenance error, because hose was mounted incorrectly (twisted).

 Contributing factors were:

 -        considering the fact that the failure was induced by fatigue, in effect by cycling the gear a large number of times, the insufficient thoroughness of the pre-flight inspections can also be a reason.

 4. Flight Safety Recommendations

 It is recommended that the S/C ,,Air Baltic Corporation”  

 4.1. Develop and implement a detailed technology for hydraulic line's  installation;  

 4.2. Perform preflight surveillance measures;

 4.3.    Evaluate and if necessary improve company Quality management system as well as perform corrective actions for prevention maintenance  deficiencies and it potential causes.

 December 29, 2006

  

 Director of Aircraft Accident

and  Incident Investigation Bureau                                                             Ivars Alfreds Gaveika