PIA AT42 near Havelian on Dec 7th 2016, engine failure
Last Update: November 18, 2020 / 16:48:21 GMT/Zulu time
Incident Facts
Date of incident
Dec 7, 2016
Classification
Crash
Cause
Engine failure
Aircraft Registration
AP-BHO
Aircraft Type
ATR ATR-42
ICAO Type Designator
AT42
Probable Primary Factors
- The dislodging / fracture of one PT-1 blade of No 1 Engine triggered a chain of events. Unusual combination of fractured / dislodged PT-1 blade with two latent factors214 caused off design performance of the aircraft and resulted into the accident.
- The dislodging / fracture of PT-1 blade of No 1 Engine occurred after omission from the EMM (Non-Compliance of SB-21878) by PIA Engineering during an unscheduled maintenance performed on the engine in November 2016, in which the PT-1 blades had fulfilled the criteria for replacement, but were not replaced.
- Fracture / dislodging of PT-1 blade in No 1 Engine, after accumulating a flying time slightly more than the soft life of 10,000 hrs (ie at about 10004.1 + 93 hrs) due to a known quality issue. This aspect has already been addressed by re-designing of PT-1 blades by P&WC.
Probable Contributory Factors
- A fractured pin (and contamination inside the OSG), contributed to a complex combination of technical malfunctions. The pin fractured because of improper re-assembly during some unauthorized / un-documented maintenance activity. It was not possible to ascertain exact time and place when and where this improper re-assembly may have occurred.
- Contamination / debris found in overspeed line of PVM of No 1 Engine probably introduced when the propeller system LRU‟s were not installed on the gearbox, contributed to un-feathering of the propeller. It was not possible to ascertain exact time and place when and where this contamination was introduced.
Important Observations
- There were several findings discovered during the course of investigation, which did not have any direct contribution to the crash / causes. However, these findings were of significant importance, and have been included as observations.
These are as follows:
- In February 2017 PIA Engineering reviewed the life of the old design PT-1 blades. PIA Engineering decided to change the soft life as a hard life of 10,000 hrs irrespective of the conditions given in the maintenance manual (an action overboard towards safe side). The enabling reasons for this review and details of participation of CAA Pakistan in this review were not recorded / provided.
- After issue of First Immediate Safety Recommendation by AAIB in Jan 2019, both PIA Engineering and CAA Pakistan (Directorate of Airworthiness) maintained the stance that the SB-21878 was not important (non-mandatory / noncritical / optional etc), contrary to the related revision in Engine Maintenance Manual (which recommends to discard the blades on completion of 10,000 flight hours when
the PT assembly or turbine disk is accessed).
- CRM training of the cockpit crew is governed by CAA Pakistan ANO ANO-014-FSXX-2.0. The refresher sessions are undertaken at prescribed periodicity (two years), by the operators by designated / qualified CRM facilitators. These trainings, were not effective, and did not yield the expected improvement in the behaviors / responses by cockpit crew. Operators as well as CAA Pakistan (Directorate of Flight Standards) did not have an effective mechanism to gauge the efficacy of the CRM trainings.
- Flight Data Monitoring (FDM) is useful tool for the operators to observe trends about the cockpit crew during regular flight operations. PIA has established an FDM analysis mechanism; however it was not being effectively utilized. In case if such systems are utilized effectively, detailed records of operational trends are established and used to feed the airline SOP and training program.
- Flight inspectors from CAA Pakistan (Directorate of Flight Standards) supervise the periodic Simulator Sessions of the cockpit crew of all operators. During the conduct of these CAA supervised Simulator Sessions, response to exposure to different situations is formally evaluated and weak areas are identified. PIA needs to undertake necessary improvements and establish a continuous monitoring system (during regular flight operations) for the identified weak areas by using suitable tools (ie FDM analysis etc).
- It was established that metal debris (small particles), likely from No 6 bearing seal of engine travelled inside OSG through contaminated engine oil. Same oil is used by Propeller Control System components (ie OSG, PVM, Feathering & SLPS solenoids etc). The OSG incorporates orifices and polyester screens protecting downstream components from contaminants too large to exit through the PVM solenoid hydraulic drain, whereas the protection valve inside PVM has wire mesh screens.
- As a redundant design, PEC „ON‟ is a secondary control for feathering as PEC commands to the PVM‟s EHV. In the AP-BHO event (engine in flight shutdown with PEC „OFF‟ (depowered) plus pre-existing independent conditions), normal feathering method using PEC command to PVM's EHV might have provided additional margin. However, an acceptable means of incorporating a specific operating procedure change, into the overall fault accommodation philosophy utilized on ATR aircraft systems, has not been identified by ATR.
- CMM of OSG has been recently revised by OEM. AAIB understands that the revised CMM must essentially encompass all conditions to rule out possibility of incorrect assembly of the lower body of the OSG and consequent damage to the pin. Furthermore it is expected that once an OSG goes through any inspection at the MRO facility, it has no hidden / latent defect.
The SIB summarized the sequence of events leading to the crash:
The aircraft remained in air for about 42 minutes before crash (all timings in UTC). These 42 minutes have been split into three stages of flight, described hereunder:
Initial Stage:
From 10:38 to 11:04 (~26 minutes) degraded speed governing accuracy of the port propeller was evident in the DFDR data, but was apparently not observed by the cockpit crew6. The flight stabilized at an altitude 13,500ft AMSL and a cruising speed of 186 knots IAS (instead of expected 230 knots IAS). There were two latent7 pre-existing technical anomalies in the aircraft (a Fractured / dislodged PT-1 blade due to a known quality issue and a fractured pin inside the OSG), and one probable latent pre-existing condition (external contamination) inside the PVM of No 1 Engine. Digital Flight Data Recorder (DFDR) analysis indicates that No 1 Engine was degraded.
Middle Stage (Series of Technical Malfunctions)
From 11:04 to 11:13 (~09 minutes), a series of warnings and technical malfunctions occurred to No 1 Engine (left side) and its related propeller control system.
These included Propeller Electronic Control (PEC) fault indications, followed by No 1 Engine power loss, and uncontrolled variation of its propeller speed / blade pitch angle (abnormal system operation). The propeller speed which was initially at 82% (cruise setting) decreased gradually to 62% and later at the time of engine power loss it increased to 102% (and stayed at that value for about 15 to 18 seconds). It then reduced down to Non Computed Data (NCD) as per DFDR. At this point, (based on simulation results) the blade pitch angle increased (possibly close to feather position). Later, the propeller speed increased to 120% to 125% (probably caused due to unusual technical malfunctions) and stayed around that value for about 40 to 45 seconds. It finally showed an abrupt drop down to NCD again. At this point, (based on simulation results) the blade pitch angle may have settled at a value, different from the expected feathered propeller. During this unusual variation of propeller speed, there were drastic variations in the aircraft aerodynamic behaviour and sounds. The directional control was maintained initially by the Auto-Pilot. A relatively delayed advancement of power (of No 2 Engine) post No 1 Engine power loss, reduction of power (of No 2 Engine) for about 15 seconds during the timeframe when left propeller rpm was in the range of 120% to 125%, and once again a reduction of power towards the end of this part of flight, were incorrect pilot actions, and contributed in the IAS depletion. Auto-Pilot got disengaged. Towards the end of this part of flight, the aircraft was flying close to stall condition. No 1 Engine was already shutdown and No 2 Engine (right side) was operating normal. At this time, IAS was around 120 knots; aircraft started to roll / turn left and descend.
Stick shaker and stick pusher activated. Calculated drag on the left side of the aircraft peaked when the recorded propeller speed was in the range of 120% to 125%. During transition of propeller speed to NCD, the additional component of the drag (possibly caused due to abnormal behaviour of left propeller) suddenly reduced. The advancement of power of No 2 Engine was coupled with excessive right rudder input (to counter the asymmetric condition). This coincided with last abrupt drop in the propeller speed11. As a combined effect of resultant aerodynamic forces aircraft entered into a stalled / uncontrolled flight condition, went inverted and lost 5,100ft AMSL altitude (ie from ~13,450ft to 8,350ft AMSL).
Final Stage
The final stage of flight from 11:13 to 11:20 (~07 minutes) started with the aircraft recovering from the uncontrolled flight.
Although blade pitch position was not recorded (in the DFDR – by design), and it was not possible to directly calculate that from the available data, a complex series of simulations and assumptions estimated that the blade pitch of left propeller may have settled at an angle around low pitch in flight while rotating at an estimated speed of 5%. Aircraft simulations indicated that stable additional drag forces were present on the left side of the aircraft at this time and during the remaining part of flight. Aircraft had an unexpected (high) drag from the left side (almost constant in this last phase); the aircraft behavior was different from that of a typical single engine In Flight Shutdown (IFSD) situation. In this degraded condition it was not possible for the aircraft to maintain a level flight. However, that level of drag did not preclude the lateral control of the aircraft, if a controlled descent was initiated. The aircraft performance was outside the identified performance envelope. It was exceptionally difficult for the pilots to understand the situation and hence possibly control the aircraft.
The aircraft came to rest at position N34.0059 E73.1778 (as identifyable through sat images of December 11th 2016 and the map of the SIB).
The SIB analysed:
Latent Pre-existing Technical Anomalies / Condition before the Flight
The flight took off at 10:38 hrs (UTC) with two latent preexisting technical anomalies inside the No 1 Engine and same side propeller system and one probable latent pre-existing condition. One anomaly was a fractured Power Turbine Stage 1 (PT-1) blade, and the second anomaly was a fractured pin inside the Overspeed Governor (OSG) of the same side. The probable latent pre-existing condition was contamination (external from the engine) observed in Propeller Valve Module (PVM).
Most probably, the PT-1 blade had fractured during previous
flight (Peshawar to Chitral), however this defect is not observable
during regular operations.
Fracture or distress of PT-1 blade may not essentially lead to an immediate IFSD, however, if it happens, (and if not combined with other independent failures) the aircraft can fly on the other engine and land.
It was determined that the pin inside the OSG was fractured due to improper re-assembly18. Metallurgical evaluation of the OSG pilot valve pin fracture surface, at Woodward USA determined that the pin had failed in overload resulting from the valve being forced together using an improper re-assembly method during some unauthorized / undocumented maintenance activity.
Analysis of complete records / history of OSG revealed that there was no reported unauthorized / un-documented maintenance activity. Since manufacturing, this particular OSG was sent to its certified maintenance facility (Woodward / Honeywell) first time in 2011, then in 2012 and lastly in April, 2015.
It was not possible to ascertain when and where unauthorized / undocumented maintenance of OSG may have occurred.
OSG can continue to be functional without any problem detected with a sheared pin of the pilot valve, until further deterioration. Continued operation with a broken pin may possibly have weakened component(s) inside OSG (ie the flyweights at the toe location).
Probable latent pre-existing contamination / debris found in PVM were most likely introduced when the propeller system LRU‟s were not installed on the gearbox. However it is not possible to ascertain when and where the contamination in the PVM was induced.
It has been established that any of the latent pre-existing technical anomalies and probable latent pre-existing condition (ie fractured PT-1 blade, or fractured pin inside OSG, or external contamination in PVM) alone may not lead to such a catastrophic / hazardous situation except in the presence of unusual combination and / or additional contributing factor(s).
Sequence of Technical Failures and Crash
The summarized sequence of the technical failures was as
follows:
Before the accident flight:
- Engine Power Turbine Stage 1 (PT-1) Blade fractured / dislodged causing imbalanced rotation of PT shaft.
- OSG pin fractured.
- Probable contamination (external from the engine) in PVM.
Prior to 11:05:31: Engine degraded and caused engine oil system contamination.
Subsequently: Propeller Control Fault
- indications and Power-plant malfunctions.
- Left OSG caused uncommanded decrease in propeller speed. This was due to the fractured OSG pilot valve pin combined with oil contamination from the engine system.
- PEC Fault triggered and crew reset and eventually permanently de-powered the PEC.
11:10:34 No 1 Engine suffered power loss.
Subsequently: Crew requested feathering, propeller speed decreased.
11:10:57 Crew positioned CL in FSO position.
Subsequently: OSG became non-functional due to loss of contact with
broken flyweights.
11:11:18 to 11:11:53
- Propeller went out of feather (Np-1 over shoot to 120%) most probably due to contamination inside the overspeed line of the PVM. This caused the protection valve to leave the protected mode, resulting in propeller movement towards low pitch below low pitch value in flight.
~11:12:30 onwards
- Sharp decrease in Np-1, blade pitch angle most likely moved further beyond the previous position (ie below low pitch in flight) and settled with Np-1 below 5% (estimated) with a drag force of about 2,000 lbf (estimated).
The aircraft crashed after 42 minutes of flight at 11:20 about 3.5
NM SSE of Havelian, and 24 NM North of BBIAP Islamabad. All 47
souls (42 passengers and 05 crew members) were fatally injured.
PIA Maintenance, Anomaly of PT-1 Blades, Latent Pre-Existing OSG Fractured Pin and PVM Contamination
The distress mode of PT-1 blades was from a known issue on P&WC “PW127” series engines since 2007. To address this issue, the OEM undertook various improvements (in the management / design of the blades). As a final effort, in October 2015 (ie ~08 years since the trending failures in the industry were being observed), the OEM introduced a new design of the PT-1 blade, through a Service Bulletin No 21878. Subsequently, the OEM amended the Engine Maintenance Manual in May 2016 (ie ~06 months prior to the crash) by specifying replacement criteria for both new and old design blades.
Past maintenance records at PIA indicated that the No 1 Engine of the aircraft was removed from another ATR aircraft (AP-BHP) during the second week of November 2016 (ie ~26 days prior to the occurrence) on a defect of rubber FOD stuck inside engine LP impeller. This was an unscheduled activity.
During shop visit, the blades had accumulated 10004.1 hrs and the PT Assembly was removed (to take out the FOD stuck inside LP impeller). Pre-conditions to replace the PT-1 blades were met as per OEM‟s defined criteria given in the revised Engine Maintenance Manual Chapter-5. However, these blades were not replaced and PIA Engine Shop cleared the engine. This engine was later installed on 16 November 2016 at No 1 position on AP-BHO.
This engine after operating for another 93 hrs on AP-BHO, had one of its PT-1 blades fractured (from a known issue). This event triggered a sequence of technical malfunctions in the event flight. However, it can be assumed that if this engine had not encountered a rubber FOD, the said PT-1 blade might have continued operating (as per OEM‟s instructions) and might have fractured around same time frame (ie 10004.1 + 93 hrs).
Fractured pilot valve pin of OSG was present since it was last accessed during a maintenance activity. It was not possible to ascertain when and where this maintenance activity took place.
Probable pre-existing contamination / debris found in PVM were most likely introduced when the propeller system LRU‟s were not installed on the gearbox. It was not possible to ascertain when and where this contamination was introduced.
Nature of Technical Malfunctions and Degradation in Aircraft Performance
In this particular single engine IFSD, coupled with a propeller
possibly rotating at 5% (estimated) rpm and a blade pitch assumed to be near (or below) the low pitch stop, the pilots came across a situation which was neither experienced earlier, nor expected. Due to system redundancy and accumulated probability of independent failures, and since the probability meets and exceeds applicable safety regulations, it was not considered as a condition to be addressed, therefore, it was not explained in any operational publication by the aircraft OEM (ATR).
Due to this combined technical anomaly, during following parts of the flight, the conditions were exceptionally difficult (ie may be considered as conditions of hazardous consequence) and it was expected that the cockpit crew may not be able to cope with the situation, and therefore they may not be relied upon to undertake the required / expected actions correctly33. These are as follows:
11:10:33 to ~11:10:56: During this part at the time of No 1 Engine IFSD, Np-1 had increased (before engine shutdown) to about 102%.
11:10:56 to ~11:11:45: Np-1 decreased and became NCD. Its behavior looked like a feather request. Then, Np-1 unexpectedly increased again at an abnormal slow rate, corresponding to propeller un-feathering.
11:11:45 to ~11:12:35: During this part Np-1 increased to a very high value range of 120 to 125 %, gradually reduced to 116.5%, and then increased to 123% again. During this part of flight the left side of the aircraft produced high drag values, until the propeller speed began to rapidly decrease in an unexpected manner.
11:12:45 to ~11:13:09: During this part the aircraft entered an uncontrolled / stalled condition of flight where the aircraft lost about 5,100ft and rolled right by 360º and beyond. This had immense psychological impact on the cockpit crew, and it impaired their capacity to perform normally.
11:12:36 to ~11:20:39: During this last part of flight when there was no further technical degradation and the blade pitch angle and Np-1 had stabilized at a particular value. This new pitch angle was possibly beyond the low pitch in flight (ie in fine pitch range normally corresponding to ground operations). The aerodynamic drag of the left side of the aircraft was estimated to be seven times more than the drag usually expected during single engine flight envelope (with the effected side propeller in feather position).
All flight parts subsequent to un-feathering (except first condition ie sub para A of para ii above) are not covered in QRH / FCOM of ATR aircraft. ATR describes the failure condition (corresponding to un-feathering and not to subsequent phases) in risk factor / safety assessment paradigm as failure condition No 1.003 “engine failure in cruise without propeller feathering” (System Safety Analysis 42.0078/95 issue 5), as of “Hazardous Consequence”, with further explanation about the possible results.
All flight parts subsequent to un-feathering (ie sub para A of para ii above) were understandably much more complicated and difficult to handle, than “engine failure in cruise without propeller feathering” (ie the first condition), and therefore are considered more severe for their possible consequence(s). Moreover, the aircraft was flying with Pitch Disconnect which probably brought in additional challenges for the aircrew in terms of aircraft handling and control authority.
The torque value of No 2 Engine during the flight conditions (sub para E of para ii above) was sufficient enough to fly, cross over the mountains and land the aircraft with No 1 Engine IFSD (if the propeller was in feather condition, and there was no additional drag due to complicated technical malfunctions of No 1 Engine propeller system).
The event was unexpected and the cockpit crew was not trained for this specific sequence of event. This event highlights importance of adhering to the cardinal principle of Fly, Navigate, and Communicate, especially in an unusual emergency situation. The crew actions indicated several events of incorrect prioritization. Top priority must always be accorded to the control of the aircraft first and then consume the remaining effort in effective management of cockpit resources for mitigation of hazards, and subsequent safe recovery of the aircraft. This aspect is however considered an overboard expectation from the pilots especially when they were unable to understand and correct the situation, and had no method available to them to reach to the correct understanding about possible descend / landing profiles (on any nearby airfield or attempt ditching elsewhere), without any specific guidelines provided in any form.
The SIB analysed that although the names of both Captain and First Officer appeared on a list of suspicious pilot licenses in 2019, the names were removed from the list "on the basis of criteria / standard being followed during the review process."
With respect to Pakistan's CAA oversight the SIB wrote:
P&W Canada identified that the reliability of PIA PW127 series engines is lower than the entire fleet operating in rest of the world. The oversight mechanism established by CAA Pakistan was found to be inadequate to identify and monitor performance indicators that can reflect such findings. Furthermore the mechanism for a proactive intervention upon such findings was in-effective.
PIA has established Maintenance Repair Overhaul (MRO) Facility for the maintenance of PW127 engine series. Such setup is authorized for the maintenance in accordance with the conditions and requirements prescribed by the respective OEM. During a site survey of the said PIA MRO facility by P&WC in April 2017, few anomalies (deviations from requirements / procedures given by P&WC) were observed44, which were not registered / documented by CAA Airworthiness during audits (or any activity related to the oversight). The oversight mechanism of CAA Pakistan (Directorate of Airworthiness) was inadequate / ineffective to identify such weak areas.
Non implementation of SB-21878 (and related deviation from relevant engine maintenance manual) was neither identified by PIA Quality and Safety Management System nor by CAA Airworthiness oversight system.
A number of IFSD cases were recorded on ATR aircraft in PIA, from 2008 to 2016 (ie before the crash). These cases and all other occurrences / incidents are mandatorily reported to CAA Pakistan. PIA Quality and Safety Management System, and the CAA Pakistan were unable to identify the trend(s) and undertake any proactive intervention.
Incident Facts
Date of incident
Dec 7, 2016
Classification
Crash
Cause
Engine failure
Aircraft Registration
AP-BHO
Aircraft Type
ATR ATR-42
ICAO Type Designator
AT42
This article is published under license from Avherald.com. © of text by Avherald.com.
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