Silk Way AN12 at Camp Dwyer on May 18th 2016, engine failure
Last Update: September 5, 2022 / 17:10:38 GMT/Zulu time
Azerbaijan's Accident Investigation Commission dispatched an investigating team to Camp Dwyer and is going to investigate (Silk Way is based in Azerbaijan). The crew consisted of an Uzbekistan citizen (Captain), 3 Ukrainians and 5 Azeris.
Antonov reported the useful life of the airframe 4K-AZ25 manufactured in 1963 was due to expire in October 2016.
Silk Way Airlines operate, amongst other aircraft types, 2 Antonov AN-12BP (4K-AZ23 and 4K-AZ63), 2 Antonov AN-12BK (4K-AZ33 and 4K-AZ93) and an Antonov AN-12B (4K-AZ25). 4K-AZ25 was seen positioning from Baku (Azerbaijan) to Bagram (Afghanistan) early May 18th 2016, she was assigned to carry cargo from Bagram to Camp Dwyer, then position to Mary for the next task.
No weather data are available.
A local resident reported (via reader comments), that were was no hostile activity whatsoever, and stated regarding weather: "Temp was approximately 43 degrees C, sky clear, unlimited visibility. A dust storm came in later, but was no factor at the time of the incident."
On Jun 8th 2016 the Interstate Aviation Committee (MAK) announced, that Afghanistan have delegated the accident investigation to the MAK, the MAK is now leading the investigation into the accident, Azerbaijan's Accident Investigation Commissions as well as specialists from Ukrainian aircraft manufacturer are participating in this investigation.
On Nov 16th 2016 Russia's MAK released their preliminary report (in English!) reporting that the first officer (35, CPL, 4,625 hours total, 836 hours on type) was pilot flying, the captain (63, ATPL, 22,628 hours total, 3,953 hours on type) was pilot monitoring. While taxiing out for departure the flight engineer reported the #3 engine (inboard right hand) was showing an MGT of above 600 and even 700 degrees, the captain requested to be more attentative. The flaps were set to 15 degrees, CG as well as gross weight was within limits. While reading the takeoff checklist the propellers were locked, #1 and #4 propellers moved into their locked position, the #2 propeller reached the position about 17 seconds after the others, there was no evidence the #3 propeller ever locked. ATC reported the winds from 280 degrees at 14 knots gusting 26 knots and cleared the flight for takeoff.
The MAK wrote:
While performing takeoff the crew first increased thrust on Engines #1 and #4 and then on Engine #2 after 10 seconds. The thrust of the three engines was about 50 kg/sq.cm as per torque indicator (lower than takeoff mode). The third engine was still operating in the ground idle mode, though the CVR did not record any crew callouts concerning Engine #3 operation parameters.
Based on the CCTV system of Dwyer aerodrome the takeoff roll was initiated almost from the RWY threshold and was conducted to the left of the RWY centerline. No significant deviations from the takeoff course during the takeoff roll were recorded. During the takeoff roll the rudder was deflected left close to extreme. Probably the pilots were also applying differential control on Engine #2 to decrease the right torque moment.
At 10:00:14 at approximately 120 km/h IAS the “Engine #3 negative thrust” signal was started to be recorded and was recorded on up to the end of the record. At that time the a/c was about 430 m away from the start of the takeoff roll.
At 10:00:42 Engines #1 and #4 thrust was increased up to 63 kg/sq.cm as per torque indicator (consistent with takeoff mode for the actual flight conditions). At that time the IAS was about 150 km/h. Engine #2 thrust was increased up to the same value only 23 seconds later at about 200 km/h IAS. At that time the a/c was about 840 m away from the RWY end. Engine #3 was still operating in ground idle mode.
Approximately 260 m before the RWY end at a speed of 220 km/h IAS (maximum speed reached) the FDR recorded the start of nose up input on the control column. The a/c did not lift off. After rolling all along the RWY the a/c overran the RWY onto the ground at a speed of 220 km/h.
While moving on the ground the aircraft sustained significant damage, which led to post-crash fire that destroyed most of the aircraft structures. Out of the nine persons on board seven were killed and two were seriously injured and taken to hospital.
After the a/c overran the RWY it collided with a metal net fence with barbed wire at a distance of 320 m from the RWY end, veering off about 80 m to the right of the RWY extended centerline. At that moment the right wing was destroyed, which resulted in fuel spill and post-crash fire.
While the a/c was moving further beyond the RWY it was further destroyed: right main landing gear and right wing were separated, Engines # 3, #4 and #1 were ripped off their mounts.
The a/c came to rest at a distance of about 600 m from the RWY end and lateral deviation of 250 m to the right of the RWY extended centerline. Most of the a/c structure was destroyed by the post-crash fire.
Some time in the past the MAK released their final report concluding the probable causes of the crash were:
The An-12B 4K-AZ25 aircraft accident was caused by the crew decision to conduct the takeoff with OEI (engine #3), the propeller of which was not feathered. In course of the takeoff run, engine #3 was kept in the Ground Idle mode, therefore the propeller was producing a negative thrust which was increasing as the speed was increasing, preventing the aircraft from reaching the liftoff speed.
The crew took no measures to abort the takeoff, and this resulted in the aircraft RWY overrun at a speed of 220 km/h, as well as in the aircraft destruction in the post-crash fire and in the deaths of people.
The MAK analysed:
After the clearance for engines' startup was received from Dwyer AD controller, the crew started the preflight preparation. According to the controller report the OAT at Dwyer AD was + 42 °С. The flight radio operator confirmed that the information was received; however, he specified that the aircraft required the opposite wind direction position for engines startup: «Roger QNH 1-0-0-8, (starting up), we need start up opposite wind direction». Considering the above request, the flight crew was prepared for difficult engines startup in high temperature conditions (with headwind, the compressor flow increases that enhances startup reliability). It must be noted that the high OAT and the significant Dwyer AD elevation (737 m) contributed to the significant difficulties on the engine startup.
Another aircraft was taxiing in. The MAK analysed:
Based on these communications' records, it follows that the other aircraft position was interfering with An-12B 4K-AZ25’s possibility to taxy and to take the needed headwind position. Starting from this moment the crew members showed some evidence of rushing in their actions.
The check-list "Before engine startup" (see Figure 8) was done in full; there were no comments from the flight crew about the airplane and/or systems' operation. However, the check of the airplane control system was done on a pro forma basis (no rudder, elevators and/or ailerons' deflections to the stop positions were recorded).
Engine #1 startup was initiated at 09:11:34 (see Figure 8), and 1 min 24 s later, the flight mechanic's report was recorded: "Engine #1 is started up, to number 4". It could be mentioned that during engines" startup, the flight crew turned the IGV heating on. "I am asking, if you have turned the IGV heating on?", "For engine #4 to turn the IGV heating on, for the number one – to turn it out". Turning IGV heating to ON before the engine startup (as well as positioning the aircraft headwind) increases the air flow through the compressor first stages, which enhances startup reliability.
After engine #1 startup was reported, the crew initiated engine #4 startup. At 09:14:20, CVR recorded the flight radio operator's report: "The starter is OFF", however, according to the decreased indications of the torquemeter, engine #4 did not start up. Further, by the Captain command: "Startup for the third, IGV of the third one – ON, of the fourth one – OFF", the crew initiated engine #3 startup. According to the FDR records, engine #3 startup also failed. In addition, due to the other aircraft’s interference, An-12B 4K-AZ25 had no possibility to take up the needed headwind position.
At 09:16:15, the crew tried to start engine #2, but this attempt also failed. Therefore the crew tried to start up the engines in the sequence defined by the Captain during the preflight preparation (1 – 4 – 3 – 2). However, they managed to start up only engine #1.
At 09:20:08 and at 09:29:53 two other attempts to startup engine #4 were recorded; both attempts also failed. Moreover, An-12B 4K-AZ25 still did not have the possibility to change the aircraft ground heading.
At 09:31:57, the crew initiated engine #3 startup (see Figure 8). According to the recorded torquemeter indications, this attempt was successful, and at 09:33:40, the flight radio operator report was recorded: "Engine #3 generators are ON to supply the aircraft". After the report, at 09:33:44, the second attempt to start engine #2 up was recorded, the attempt failed. At 09:36:16, the crew requested the ATC controller’s clearance for taxiing for obtaining the headwind position.
After ATC controller's clearance was received, the crew conducted the taxiing, and at 09:38:30 managed to take up the opposite wind position. Navigator: "Right now [we are] opposite the wind". Following the Captain command, at 09:39:00, the crew tried to start up engine #4 (see Figure 8). As before, this new attempt also failed. After that, at 09:40:55, the crew attempted to start up engine #2. Despite the change of the aircraft ground position, engine #2 also failed to start up.
The crew continued to start up engines #4 and #2. At 09:44:56, another increase of the engine #4 torquemeter indications was recorded, which shows another attempt to start up engine #4 (see Figure 8). This time engine #4 was started up, and at 09:47:22, the flight mechanic's report was recorded: "Number 4 is started, number 2 – to start up". At 09:47:46, the last engine (engine #2) was started up.
Therefore, engine #1 was started up from the first attempt, engine #2 was started up from the fourth attempt, engine #3 was started up from the second attempt, and engine #4 was started up from the fifth attempt. The sequence of the engines startups was as follows: 1 – 3 – 4 – 2.
Minimum intervals between the engine startups was about 4 min 45 s (for engine #2), about 15 min 45 s (for engine #3), about 3 min (for engine #4). The minimum intervals between engine startups were within the limits set by the An-12 AFM.
After the engines had been started up, the crew requested the ATC controller’s clearance for RWY 23. Flight radio operator: "Yard Bird NAG33CU request runway 2-3 for departure we are ready". The controller gave the clearance for taxiing: "NAG33CU [illegible] runway 23 full, back taxi line up and wait if you want you can taxi slow down G then [illegible] F". On receiving the clearance, the crew started the taxiing.
The taxiing was initiated at 09:54:31. While taxiing the crew did "At taxiing" checklist.
"At taxiing" checklist was read out in full; after the checklist was done, the crew started "At holding" checklist. "At holding" checklist was read out in full also; there were no crew remarks related to the aircraft and systems operation recorded. According to the reports, flaps were 15°, the air bleed for the engine IGV heating was OFF; the crew reported the check of autofeathering system based on the torquemeter oil pressure indication.
The check of the airplane control system was done on a pro forma basis (no rudder, elevators and/or ailerons' deflections to the stop positions were recorded).
During the aircraft taxiing, at 09:57:56, the flight mechanic reported the gas temperature increasing above the limit on engine #3: "Engine #3, see that, the engine temperature over six hundred, over seven hundred". The first officer confirmed: "Yes, it has a temperature". The Captain requested them to keep their eyes open. Immediately prior to the flight mechanic's report, the engine #3 rpm decreasing from 8.9 kgf/cm2 to 3.7…5.4 kgf/cm2 by torquemeter (which corresponds to the "Ground Idle" mode) (see Figure 8). When the flight mechanic was reporting, the aircraft was at the beginning of RWY performing a turn to the takeoff heading position.
In answer to the crew request, at 09:59:38, ATC controller gave a clearance for takeoff "...33CU runway 2-3 wind 2-8-0 (degrees at 1-4 knots) gusts 2-6 cleared for takeoff". When giving the clearance, the controller provided the crew with RWY wind information which was 280°, 14 kt (7 mps), gusts 26 kt (13 mps). Therefore, the wind was cross head, a headwind component could have been about 5…9 mps. After the clearance was received, the crew initiated the takeoff at 09:59:43 (see Figure 10).
MSRP-12-96 FDR system records show that at the beginning of the takeoff run, the torquemeter indicated the increase of oil pressure for outboard engines (engine #1 and #4) from 10…13 kgf/cm2 up to 33…36 kgf/cm2 (it means from about 0.4 of the nominal value to about 0.6); and 10 s later, the increase of the oil pressure for engine #2 up to 34 kgf/cm2. During the takeoff, the oil pressure indicating by the torquemeter for engine #3 was 3.7…5.4 kgf/cm2 which meets a Ground Idle mode. The engines' separate throttle control at takeoff shows that the crew members were aware of the problems with engine #3; they were trying to prevent the aircraft from a noseright moment caused by engine #3 malfunction by decreasing the engine #2 rpm. However, no crew comments related to engine #3 malfunction were recorded by CVR. During the takeoff run, the angle of the rudder deflection to the left almost reached the limits.
As stated above, the crew made a decision to conduct a takeoff with inoperative engine #3.
Most probably, this decision had been made considering that at the Dwyer AD there were no conditions necessary for the engine overhaul or change. No crew discussions related to the said issue were recorded by the interphone set (the CVR system design does not include an open mike and, thus, the intercabin communications were not recorded).
During the next 20 s, the rpm of the three engines (engines #1, #2 and #4) simultaneously went up to 48…54 kgf/cm2 by the torquemeter indications which corresponds to the value of 0.7 of the nominal. The engine #3 oil pressure was still about 3.7…5.4 kgf/cm2 by the torquemeter indications.
According to the Dwyer aerodrome CCTV records, the aircraft takeoff run was initiated right from the RWY threshold (see Figure 11). The takeoff run was conducted slightly to the left from the RWY centerline; during the takeoff run no significant course deviations were recorded with CCTV.
At 10:00:14, the MSRP-12-96 system started to record the "Engine #3 negative thrust" single command which was ON till the end of records ("Engines negative thrust" single command is generated when the negative thrust value exceeds 1200 kg). At that moment the aircraft was at 450 m from the RWY threshold (see Figure 11).
The generation of the single command related to the engine #3 negative thrust evidences that the said engine propeller was not feathered. Moreover, the torquemeter indications of the said engine were kept at values of the ground idle.
During the takeoff run, the autofeathering failed to occur on the inoperative engine #3. An autofeathering system puts propeller blades into the feathered position and cuts the fuel supply in case of any engine or powerplant systems’ malfunctions causing the oil pressure drop below the 10±0.5 kgf/cm² by the torqumeter indications if, prior to the malfunction, the engine has been operating with 56±2° by the throttle position indicator. Autofeathering may also occur in case of negative thrust developing up to 1200 kgf or above if, prior to the malfunction, the engine has been operating with 40±2° by the throttle position indicator. During the taxiing and takeoff run, engine #3 was not set to the temperature mode above 40° by the throttle position indicator (the recorded torquemeter indications evidence this (see Figure 10), thus, the propeller could not be feathered automatically during the takeoff run.
During the takeoff run no crew reports related to the abnormal engine #3 operation were recorded.
Thus, the aircraft takeoff run was conducted with the unfeathered propeller of the inoperative engine #3 which caused a significant negative thrust development and inhibited the speed increase.
This conclusion is also proved by the mathematical model made by ANTONOV State Company. According to ANTONOV State Company conclusion, the model results that more accurately meet the recorded aircraft trajectory parameters are the results of the scenario when there is no autofeathering of the engine #3 propeller as an answer to the reverse thrust alert. The engine #3 propeller turns into windmilling with the negative thrust development which increases as the speed increases. The situation was caused by not setting the engine #3 throttle control lever to 40±2 degrees temperature mode. Moreover, according to ANTONOV State Company conclusion, it should be assumed that the propeller of engine #3 (inoperative engine) was not latched to the flight stop.
During the takeoff run, the unfeathered propeller created the maximum negative thrust which made the takeoff impossible. The available data do not provide the opportunity to precisely determine the reason why during the takeoff run, the crew did not increase the engine #3 rpm up to the values more than the ground idle. The most probable reason may be the increase of the turbine outlet temperature of engine #3 above the maximum allowable taxiing limit. Probably, the crew was afraid that the increase of the inoperative engine rpm may cause the recurrent increase of the turbine outlet temperature above the allowable limit.
After the "Engine #3 negative thrust" alert generating, at 10:00:19, the engine #1 rpm and engine #4 rpm were increased to the torquemeter indications of 61…65 kgf/cm² (which corresponds to the takeoff mode for the actual conditions according to ANTONOV State Company's calculations) while the engine #2 rpm was decreased from 50 kgf/cm² to 45 kgf/cm² by the torquemeter. At this moment, the IAS was 150 kmph (see Figure 10).
23 s later, at the IAS of 200 kmph (10:00:42, with the takeoff run distance of about 1680 m) the engine#2 rpm was increased up to engines #1 and #4 rpm.
At 10:00:52, at a distance of 2230 m from the starting point (about 210 m before the RWY end), at a speed of 220 kmph, the initiation of the control column pulling was recorded. The crew’s actions during the takeoff run did not meet the An-12 AFM recommendations for an OEI takeoff.
The recorded information and the CCTV data of Dwyer AD evidence that with the elevator nose-up deflection to -14°, the aircraft did not lift off. On running along all the RWY length (2439 m), and the concrete area of 90 m at a speed of 220 kmph, at 10:00:57, the aircraft ran over the RWY and reached the ground. The total run before the aircraft overran the RWY concrete surface lasted 70 s. During the takeoff run the crew took no measures to abort the takeoff.
During the ground run, the aircraft received significant damage causing a fire as a result of which most of the airframe was destroyed. Out of nine persons on board seven were killed and two were seriously injured.
This article is published under license from Avherald.com. © of text by Avherald.com.
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