Caribbean AT72 at Port of Spain on May 4th 2014, fracture of propeller trunnion pin
Last Update: September 11, 2019 / 17:23:03 GMT/Zulu time
Incident Facts
Date of incident
May 4, 2014
Classification
Report
Airline
Caribbean Airlines
Flight number
BW-1861
Departure
Tobago, Trinidad and Tobago
Destination
Port of Spain, Trinidad and Tobago
Aircraft Registration
9Y-TTC
Aircraft Type
ATR ATR-72-200
ICAO Type Designator
AT72
Following the flight maintenance examined the engine, found a broken generator shaft and replaced the AC generator. Both engines and propellers were tested at 100% Np with no vibrations or other anomaly observed, the aircraft was returned to service.
The following day the aircraft performed a flight, after landing upon returning the power levers from flight to ground idle the crew noticed strong vibrations again.
Several maintenance actions did not reveal any anomaly. Both PECs were reset and the right hand PEC calibrated, however, thereafter a test run was interrupted when the right PEC went into fault mode. The propeller valve module was replaced, another test showed no vibrations and no anomaly, however, when the power levers were placed into beta, strong vibrations appeared and both engines are immediately being shut down. A following inspection revealed two of the right hand propeller blades had remained in their beta positions while the other blades had moved into feather position upon shutdown.
The French BEA released their final report in French 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 French 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).
The report concludes:
Seven cases of vibrations on ATR-72-212A have been observed over the last years. In almost all cases the fracture of the trunnion pin of a blade and damage to the actuator front plates were found. The investigation highlighted the existance of alternating overloads causing damage to the plates and one final overload in one direction which results in the facture of the trunnion pin.
The origin of the overloads and the exact chronology of damages and vibrations could not be determined. Nonetheless, these factors might be able to contribute:
- a retention force generated by friction phenomena at the blade root bearings (ball bunching)
- significant impact forces on the trunnion pin due to cyclic effects on the front plate that occur at aircraft speeds near Vmo and power levers in the idle position
- unforeseen response by the propeller control loop when the propeller control is disturbed by cyclic stress phenonena on the front plate and by friction
- maintenance actions carried out on 9Y-TTC as result of the vibrations did not identify these issues.
Due to complex technical report, available only in French, a further summary appears to be impossible (causing more confusion due to difficulty of translation). However, the final report contains an English highly technical investigation report in Annex 13 starting page 103 explaining the possible mechanism causing the trunnion pin to fracture.
On Sep 11th 2019 the BEA released the English Version of the final report concluding the probable causes of the incident were:
Seven vibration phenomena on the ATR 72-212A have been reported in the last few years.
In almost all of the cases, the rupture of a trunnion pin of one of the blades and damage to the propeller blade actuator forward yoke plate were observed. The investigation revealed the existence of alternating overloads causing damage to the yoke plates and of a final overload in one direction resulting in the rupture of the trunnion pin. It was not possible to determine the cause of these overloads and the precise chronology of the damage and vibrations. Nevertheless, several elements may have contributed to it:
ˆ
- A friction force caused by a ball bunching phenomenon.
- Significant loads caused by the trunnion pin striking the ear of the plate on forward yoke plate cyclic loading appearing when the aeroplane speed was close to VMO and the power levers in the flight idle position.
- Unplanned operation of the control loop of the propeller pitch change mechanism affected by forward yoke plate cyclic loading and friction.
The maintenance operations carried out on 9Y-TTC following the vibration phenomena did not identify this damage.
The BEA analysed:
Scenario of propeller vibration events for 9Y-TTC
Propeller related vibrations felt by crew during previous flights
During the months prior to the incident, crews of the ATR 72-212 registered 9Y-TTC had reported three propeller related vibration events:
- On 25 January 2014, the crew had felt vibrations during the descent and PEC FAULT then PEC SINGLE CHANNEL warnings associated with the right propeller had appeared.
ˆ- On 15 March 2014, the crew had felt severe vibrations during the flight when they moved the power levers to the flight idle position. The PEC FAULT warning associated with the right propeller appeared. Reading the right propeller PEC fault codes revealed an operating fault of the propeller pitch angle position sensors of the PEC primary and back-up channels (fault codes 67 and 68). Propeller vibrations can generate this type of failure.
- On 28 March 2014, the crew had felt severe vibrations during the flight when they moved the power levers to the flight idle position. The vibrations disappeared when the power levers were moved forward. The PEC FAULT warning associated with the right propeller appeared.
Maintenance teams carried out ground engine tests following each of these events, the tests detected nothing abnormal
The possibility of damage to the propeller pitch change mechanism before the flight of 4 May 2014 cannot be ruled out. Nevertheless, the analysis of the parameters recorded in the QAR showed that this damage, if it existed, was not significant. For the previous flights, the left and right engine and propeller parameters were similar when using reverse, even in the cases where there was maximum power.
Vibrations during flight on Sunday, 4 May 2014
During the descent, at an increasing speed of 246 kt close to the VMO (250 kt), the crew reduced power to the minimum by putting the levers in the flight idle position. The torques of the two engines reached zero or negative values which indicated that the propellers were windmilling. The crew then felt strong vibrations.
Given the strength of the vibrations, the right propeller pitch angle position sensors probably sent a sufficient number of out-of-tolerance values to the PEC to trip the PEC FAULT warning. The vibrations also led to the rupture of the drive shaft of the right engine AC wild generator. The intensity of the vibrations subsided when the decreasing speed reached 236 kt and the crew started the PEC fault management procedure by putting the right propeller condition lever in the “100 % OVRD” notch. The values recorded in the QAR showed a reduction in the number of pitch angle erroneous values. Finally, three or four seconds later, the vibration level had sufficiently dropped for these values to become constantly valid and the PEC FAULT warning went off.
The vibrations lasted for around 20 seconds during which time the PEC stayed in FAULT mode. It was not possible to conclude with certainty that the ears of the actuator forward yoke plate were damaged during this vibration episode. This damage could in fact have been caused during the last engine test on 5 May. However, there were numerous similarities with the vibration episode in the incident on 30 November 2014 concerning ATR SE-MDB where it was shown that the damage had occurred during the in flight vibrations.
The crew then continued the procedure by restarting the right PEC, by putting the right propeller condition lever in the “AUTO” notch and then balancing the power of the two engines.
After restarting the right PEC, an anomaly, the cause of which has not been determined, appeared on its primary channel which tripped the ENG 2 PEC SGL CH warning.
Continuation of flight on Sunday, 4 May 2014
As the actuator forward yoke plate is only loaded in flight in very specific configurations, the crew was able to continue their flight normally with a possibly deformed right propeller forward yoke plate.
However, until landing, the recorded parameters showed the pitch of the right propeller as being 2° to 5° more than for the left propeller. The rupture of the drive shaft of the AC wild generator was probably the cause of this difference. As the damaged generator was no longer absorbing its share of the power provided by the engine, the power transmitted to the right propeller was in fact greater than the power transmitted to the left propeller. Consequently, in order to maintain the same propeller rotation speed, the pitch change mechanism controlled a greater blade angle on the right propeller.
During the landing, the crew reported that they felt slight vibrations. Reverse was not used. During the taxiing and up to engine shutdown, the recordings showed a torque difference between the right and left engine in order to keep the same propeller rotation speed. The right engine provided 10 % more torque than the left engine. The cause of this difference in torque was not determined.
Maintenance operations carried out after flight of 4 May 2014
The maintenance documents showed that after the flight, the PEC fault codes of the left engine were checked and the PEC reset when the checks should have concerned the PEC of the right engine. The investigation was not able to determine if this was an entry error in the maintenance documents and what fault codes were recorded in the PEC memories.
A test with take-off power and a propeller rotation speed of 100 % Np of the two engine/propeller assemblies did not reveal vibrations or abnormal operation. However, this engine operating range does not load the actuator forward yoke plate and, consequently, does not allow its condition to be checked. The blade trunnion pins only come into contact with the forward yoke plate during two flight phases: reverse and propeller feathering(70).
The complete engine performance ground tests include a transition to reverse with maximum power. This test may have revealed the existence of damage to the right propeller pitch change mechanism but it was not carried out.
Vibrations during flight on Monday, 5 May 2014
The flight proceeded normally. The recorded parameters were normal with no difference in values between the left engine and the right engine, unlike the end of the previous flight. The replacement of the AC wild generator of the right engine before the flight was probably why this difference disappeared.
During the landing run, the crew nevertheless heard a loud noise when they moved the power levers from the flight idle to ground idle position. The recorded parameters confirmed abnormal operation of the right propeller: a decrease in the speed of the right propeller compared with the left propeller and, at the same time, an increase in the torque of the right engine of up to 17 % more than the left engine.
Thereafter, the right engine provided 10 % more torque than the left engine up to its shutdown. The cause of this difference in torque, also present during the previous flight, was not determined.
The crew reported that the noise disappeared when the right propeller was feathered. No warning appeared in the cockpit. The crew continued taxiing to the apron using the left engine only.
Maintenance operations carried out after flight of Monday, 5 May 2014
In a first ground test of the use of reverse, the maintenance agents did not feel any abnormal vibration. The data recorded by the QAR showed that during this first test, the maintenance agents twice set the power levers to the reverse position for a period of less than three seconds. In this lapse of time, the right engine did not have the time to supply maximum power and the right propeller blades were not able to reach a sufficient angle for the trunnion pin of blade 3 to pass behind the actuator forward yoke plate on which ear 3 was probably already deformed.
A second and third test were interrupted by the appearance of the PEC FAULT warning before the end of the unfeathering sequence of the right propeller. The maintenance agents had calibrated the right PEC before the second test and then replaced and calibrated the right PEC before the third test. They reported that they felt no abnormal vibration.
The right PVM was replaced. A fourth ground test was started, during which were tested ground idle, flight idle and sufficient power to reach 100 % Np without reaching take-off power, without any warning occurring or any abnormal vibration being felt. This engine operating range does not load the actuator forward yoke plate and, consequently, does not allow its condition to be checked. The power levers were then put in the reverse position. The maintenance agents felt abnormal vibrations and shut down the engines.
The pitch angle reached during this last use of reverse was sufficient for the trunnion pin of blade 3 of the right propeller to pass behind the actuator forward yoke plate. The blade 3 counterweight arm came into contact with that of blade 4, locking the pitch angle of the latter blade. When the maintenance agents tried to feather the right propeller, this was prevented by blade 4, which was still blocked. Its trunnion pin, bearing on ear No 4 of the forward yoke plate, blocked the movement of the pitch change actuator. Then, after around ten seconds, the power supplied by the actuator was sufficient to break the blade 4 trunnion pin. The actuator then moved to the feathering position. When trunnion pin 4 failed, blade 4 became free to rotate and its counterweight interacted with that of blade 5.
After shutting down the engines, blades 1, 2, 5 and 6 were feathered whereas blade 3 (whose trunnion pin had passed behind the actuator yoke plate) and blade 4 (free to rotate, trunnion pin broken) were in unusual positions.
This incident showed that despite the maintenance procedures implemented, it was possible to perform a flight with significant damage to the propeller pitch change mechanism. With this in mind, all the elements established during the analysis of similar events need to be examined to try and determine a scenario in which the propeller pitch change mechanism is damaged, and to determine the cause.
The BEA warned in their analysis:
An error by the crew in identifying the propeller concerned, leading to the shut down of the engine corresponding to the propeller in good condition, could cause a substantial loss of altitude and, if the event should occur at a low height, difficulties in maintaining the desired flight path. Indeed, the conditions for vibrations to appear in flight can simultaneously occur in flight phases at a low height. The statements given by certain pilots who said that they had encountered abnormal vibrations in a glide path final approach and in a visual approach seem to confirm this.
It does not seem to be easy for crews to identify the propeller concerned. A crew in fact first feathered the sound propeller as they had difficulties in reading their instruments. In another case, the crew reported that they had had difficulties in identifying the propeller concerned and that the vibrations increased when they reduced power on the damaged propeller.
The BEA analysed the safety measures taken so far:
2.2.3 Analysis of effectiveness of safety measures taken
2.2.3.1 Pilot information
No new incident has been reported by operators since the incident of 30 November 2014.
The installation of new bearing separators on less than a quarter of the fleet (at the time of writing this report) cannot explain in itself why this phenomenon has not reoccurred. It is possible that the pilots, informed of cases of vibrations suddenly appearing in flight by the publication of the EASA SIB and ATR AOM, avoided reducing power at flight idle at a speed close to VMO and that this contributed to it disappearing. It is probable that this raised awareness will not last over time. It could be perpetuated by more clearly integrating the manufacturer’s recommendation to avoid descents at a speed of more than 240 kt, in the operator’s operational documentation.
2.2.3.2 Operational procedures
The day of the incident there was no specific procedure linked to the appearance of vibrations in the propeller-engine assembly. Nevertheless, the crew could use the emergency procedure linked to severe damage to an engine in flight. However, this procedure does not explain how to identify the engine concerned and could lead to the sound propeller being feathered and the sound engine being shut down.
The incident concerning PK-WFV on 18 September 2013 in Indonesia, illustrates that moving the power levers does not always allow the crew to easily distinguish the propeller concerned. The crew had in fact had difficulties in identifying the propeller concerned as the vibrations increased when they reduced power.
On 23 February 2015, a procedure linked to damage to the propeller pitch change mechanism was introduced. Initially, the pilot is asked to move the power levers one after the other and to observe changes in the vibrations in order to try and distinguish which propeller is concerned. If this step is not conclusive, the pilot is then asked to first feather the right propeller and then if the vibrations continue, to unfeather the right propeller and feather the left propeller.
The incident to SE-MDB on 30 November 2014 in Sweden, shows that the damage caused to the propeller and engine may get worse during the implementation of this new procedure and in particular, that the feathering of the propeller concerned takes time. The actions carried out by the crew corresponded to this new procedure and the damage observed on the engine was substantial: engine compressor casing found split over 180° and damage to the dampers of the engine mounting brackets.
2.2.3.3 Maintenance procedures
Preventive and corrective maintenance should allow defects or damage on the propeller pitch change mechanism to be identified.
At the time of the incident, there was no specific troubleshooting procedure following an abnormal vibration event occurring in flight. In view of the difficulties encountered by the maintenance organizations to determine the cause of the vibrations in flight, it was recommended to complete a form and to send it to ATR for analysis.
The maintenance documents relating to 9Y-TTC show that after each vibration event, a ground engine test was carried out. When the vibrations were associated with a PEC FAULT warning, the maintenance personnel calibrated the PEC and, in certain cases, read the fault codes saved in the PEC memory. Pilots at the operator concerned by the SE-MDB incident stated that the maintenance department had been informed of the regular appearance of vibrations in flights and that maintenance agents had flown in the cockpit to try and observe the phenomenon. After each event, the aeroplanes were returned to flight without the cause of the vibrations being determined and without checking the actual condition of the pitch change mechanism.
In October 2014, ATR published a specific maintenance procedure to prevent an aeroplane which may have a significantly damaged propeller pitch change mechanism from being returned to flight. It is carried out according to the content of the air safety reports (severe vibrations associated with the appearance of a PEC FAULT warning) and fault codes in the PEC memory. It consists of a manual check for possible significant deformation of the actuator yoke plates. However, this procedure depends on the pilot’s subjective assessment of the intensity of the vibrations felt and does not permit the detection of the onset of deformation of the yoke plates or damage without rupture of the blade trunnion pins. In addition, without significant damage to the pitch change mechanism, the procedure does not permit the cause of the vibrations to be determined or prevent them from reoccurring in flight. Lastly, it only applies when severe vibrations occur in flight leading to the activation of the specific PEC warning.
2.2.3.4 Certification of propeller system
The tests carried out during the propeller certification campaign in 1994-1995 did not reveal certain phenomena observed during the flight tests in 2014 and 2016: the ball bunching, the forward yoke plate cyclic loading phenomenon when the aeroplane is descending at a speed close to VMO with the power levers in flight idle, the loads on the forward yoke plate when this phenomenon is present, with the intensity of the loads caused by the impact of the trunnion pin on the yoke plate ear increasing, according to the propeller manufacturer, with the gap between the actuator forward and aft yoke plates. Certain choices and hypotheses led to this situation.
First of all, the vibration measurements were only made on the left engine in the certification flight tests in 1994-1995. Due to the aeroplane’s design, the left propeller was in fact considered to be the most loaded of the two in operation. The measurements made on the left propeller therefore seemed conservative with respect to the right propeller. However, the flight tests carried out in 2014 and 2016 showed that the right propeller was more sensitive to certain vibration phenomena than the left propeller.
Next, despite the malfunctioning of the sole sensor positioned on the trunnion pin of a left propeller blade, it was decided not to carry out new flight test campaigns. Finally, as the blade counterweights were designed so that the forward yoke plate of the propeller pitch change mechanism is not generally loaded in flight, only the study of the static loads applied to the blade trunnion pin in climb and cruise was envisaged, and not its vibration behaviour in descent, a flight phase where the static loads applied to the aft yoke plate are theoretically lower.
The FAA published several circulars proposing a method for assessing the vibration stresses borne by a propeller within the scope of its certification. At the time of the propeller’s certification, the FAA circular 20-66 did not explicitly recommend performing a descent at flight idle with different speeds.
Since 17 September 2001, circular 20-66A (replaced by 20-66B in 2011) recommends in particular, to perform descents at flight idle with different speeds during the flight tests. The systematic implementation of this type of check with different speeds around VMO, including speeds slightly above VMO, could make it possible to confirm that the propeller design has sufficient margins before the appearance of vibration phenomena such as that observed during the incidents.
Incident Facts
Date of incident
May 4, 2014
Classification
Report
Airline
Caribbean Airlines
Flight number
BW-1861
Departure
Tobago, Trinidad and Tobago
Destination
Port of Spain, Trinidad and Tobago
Aircraft Registration
9Y-TTC
Aircraft Type
ATR ATR-72-200
ICAO Type Designator
AT72
This article is published under license from Avherald.com. © of text by Avherald.com.
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