American B763 at Chicago on Oct 28th 2016, rejected takeoff, fire at right hand wing due to uncontained engine failure

Last Update: February 6, 2018 / 18:04:36 GMT/Zulu time

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Incident Facts

Date of incident
Oct 28, 2016

Classification
Incident

Cause
Rejected takeoff

Airline
American Airlines

Aircraft Registration
N821NN

Aircraft Type
Boeing 767-300

ICAO Type Designator
B763

On Feb 6th 2018 the NTSB released their final report concluding the probable causes of the accident were:

The National Transportation Safety Board determines that the probable cause of this accident was the failure of the high-pressure turbine (HPT) stage 2 disk, which severed the main engine fuel feed line and breached the right main wing fuel tank, releasing fuel that resulted in a fire on the right side of the airplane during the takeoff roll. The HPT stage 2 disk failed because of low-cycle fatigue cracks that initiated from an internal subsurface manufacturing anomaly that was most likely not detectable during production inspections and subsequent in-service inspections using the procedures in place.

Contributing to the serious passenger injury was (1) the delay in shutting down the left engine and (2) a flight attendant’s deviation from company procedures, which resulted in passengers evacuating from the left overwing exit while the left engine was still operating. Contributing to the delay in shutting down the left engine was (1) the lack of a separate checklist procedure for Boeing 767 airplanes that specifically addressed engine fires on the ground and (2) the lack of communication between the flight and cabin crews after the airplane came to a stop.

The NTSB analysed the accident sequence:

The right engine failure occurred when the CVR recorded a “bang” sound, at which time the airplane’s airspeed was 128 knots. Both flight crewmembers then felt the airplane drift to the right, and the captain thought that the airplane was unsafe to fly. At that time, no warnings were being annunciated in the cockpit to indicate to the flight crew that an uncontained engine failure had occurred or that a fire had subsequently begun. The captain moved the throttles to the idle position when the airspeed was 130 knots and began the rejected takeoff maneuver. The start of the rejected takeoff maneuver occurred 1.2 seconds after the “bang” sound. The RTO autobrakes activated when the airspeed was 134 knots, which was also V1 for the flight. According to the American Airlines B767 Operations Manual, QRH, the decision to reject the takeoff must be made in time to start the maneuver by V1.

The first officer informed the tower controller that the takeoff was being rejected, and the tower controller notified the flight crewmembers of a fire, which was their first indication of the fire. The airplane stopped on the runway about 26 seconds after the “bang” sound. The NTSB concludes that the captain made a timely decision to reject the takeoff and performed the maneuver in accordance with company training and procedures.

The uncontained engine failure resulted from an HPT stage 2 disk rupture (rotor burst). The HPT stage 2 disk was recovered in four fragments (shown in figure 5 in section 1.3.3). One disk fragment and other exiting engine debris fragments (for example, blades, static hardware, or HPT stage 2 disk posts) impacted and penetrated through the inboard section of the right wing, creating two separate holes (shown in figure 7 in section 1.3.3). Disk fragments “B,” “C,” and “D” were recovered outboard of the right engine (to the north of the airplane); as a result, they could not have caused the damage observed inboard of the engine.

In addition, disk fragments B, C, and D were likely part of a single disk piece that had fractured into three separate fragments after impacting the runway. Specifically, an impact ground scar/gouge was observed about 32 ft to the right of the runway 28R centerline (outboard of the right engine location). This ground scar/gouge was consistent with impact from a single HPT stage 2 disk fragment. Thus, this disk fragment was most likely intact as it exited the right engine and nacelle and impacted runway 28R, creating the observed ground scar/gouge and fracturing into three pieces. The overstress fractures on disk fragments B, C, and D were consistent with a hard impact, which would be expected given the exit velocity, size, and weight of the disk fragment before it fractured.

Disk fragment “A” was recovered about 2,935 ft south of the uncontainment location at a UPS building. Hole 1 (located in the inboard section of the right wing forward of the front spar) was too small for disk fragment A to fit through, and the edges of the hole exhibited tearing and ripping that were consistent with impact from smaller debris with less momentum, such as that from exiting engine debris fragments. The trajectory of the released debris showed that the debris did not impact the front spar but instead continued upward and exited the top of the leading-edge panel.
The trajectory for the disk fragment causing the damage to the inboard section of the right wing aft of the front spar (hole 2) showed that the fragment entered the dry bay through the lower wing skin, severed the main engine fuel feed line, severed rib No. 6 (part of the inboard dry bay boundary), and continued through the upper wing skin.74 To create this damage, a large disk piece with significant energy would be needed. Disk fragment A weighed about 57 pounds and represented more than one-third of the disk. Thus, hole 2 was created by disk fragment A of the HPT stage 2 disk.

Disk fragment A did not cause the impact damage observed on the fuselage, landing gear doors, and left engine nacelle; the cause of that damage was consistent with smaller exiting engine debris fragments. If disk fragment A, which was found intact, had struck the fuselage, landing gear doors, or left engine nacelle, the fragment would have created a sizeable hole due to the exit velocity, size, and weight of the fragment. Also, Boeing’s trajectory analysis, which estimated the exit angle for the disk fragment that departed the right engine and penetrated through the right wing, determined that the disk fragment exited the engine about 4.3° aft of the HPT stage 2 disk rotation plane and that the fragment passed over the fuselage.

The NTSB concludes that the right engine experienced an uncontained HPT stage 2 disk rupture during the takeoff roll. The HPT stage 2 disk initially separated into two fragments. One fragment penetrated through the inboard section of the right wing, severed the main engine fuel feed line, breached the fuel tank, traveled up and over the fuselage, and landed about 3,000 ft away. The other fragment exited outboard of the right engine, impacting the runway and fracturing into three pieces.

Postaccident observations of the HPT stage 2 disk fragments found that the fracture surfaces in the forward bore region of the disk exhibited a heat-tinted appearance. High-magnification optical examination of the fracture surfaces revealed the presence of dark gray subsurface material discontinuities with multiple cracks initiating along the edges of the discontinuities. Examination of the material underneath the largest discontinuity, which was about 0.43 inch in length and 0.07 inch in width and was inclined about 26° to the bore centerline, revealed a discrete region underneath the discontinuity that appeared white compared with the surrounding material. Surrounding and interspersed within this region were stringers of micron-sized oxide particles referred to collectively as a “discrete dirty white spot.”

The multiple cracks that had initiated along the edges of the discontinuities propagated radially inward toward the bore and outward toward the disk rim and blade slots. Under SEM examination, the cracks exhibited striations that slowly increased in spacing as the crack length increased, which was consistent with low-cycle fatigue.76 (Outside of the heat-tinted region, the fractures exhibited features consistent with overstress.) The NTSB concludes that the HPT stage 2 disk failed because of multiple low-cycle fatigue cracks that initiated from an internal material anomaly, known as a discrete dirty white spot, which formed during the processing of the material from which the disk was manufactured. Section 2.4 provides more information about the HPT stage 2 disk failure.

The NTSB analysed the engine fire checklist critically:

The flight crew received aural and visual engine fire warnings after the captain’s decision to reject the takeoff. Given these warnings, the flight crew began the 16-step engine fire checklist, the first 5 steps of which were memory items. Step 3, which was accomplished about 4 seconds after the captain called “checklist,” instructed the flight crew to cut off the fuel control switch on the affected side (in this case, the right side), which shut down that engine. Step 5 instructed the flight crew to rotate the engine fire switch (shown in figure 15) to its stop and then wait 30 seconds to see if the engine fire warning light remained illuminated. If the light was still illuminated, the engine fire switch was to be rotated to its other stop.

FDR data showed that the handle for the right engine fire extinguisher was pulled about 8 seconds after the captain called “checklist.” About 23 seconds after the handle was pulled, the captain asked the first officer if he had discharged the fire extinguisher bottle, and the first officer replied that he “pushed it twice.” The first officer then realized that he had pulled but not rotated the engine fire switch, at which time he accomplished that step.

Because of the wind at the time (from 180° at 10 knots) in relation to the airplane’s location on runway 28R, the smoke was blowing away from the cockpit, and the flight crew could not readily see the amount of smoke coming from the right engine. Also, postaccident observations in an exemplar 767 airplane demonstrated that the first officer would not have been able to see the right engine and most of the right wing when looking out the cockpit right-side window from his seat. Thus, at that point, the flight crew was unaware of the severity of the fire.80 The flight crew was also unaware that the cabin was beginning to fill with smoke.

About 4.5 seconds after the first officer discharged the fire extinguisher bottle in the right engine, the captain stated, “oh look at the smoke—check out the smoke.” The captain stated, during a postaccident interview, that he recognized that continuing the engine fire checklist would not be appropriate because the airplane was on the ground. He called for the evacuation checklist, which had nine steps, about 4 seconds after seeing the smoke. The captain stated that, while performing the evacuation checklist, he heard commotion outside the cockpit door and realized that the cabin was being evacuated. The fourth step of the evacuation checklist instructed the flight crew to cut off the fuel control switches to shut down both engines.

Although the right engine had already been shut down as part of the engine fire checklist, shutting down the left engine did not occur until the flight crew depressurized the airplane (the third evacuation checklist step), which the captain reported took a long time, even though the airplane had not yet been in flight.81 (The NTSB notes that, at this point, the exits would have been opened, so the cabin would have already been depressurized.) FDR data showed that the left engine was shut down about 59 seconds after the airplane came to a stop, and video evidence showed that the left engine spooled down about 10 seconds later.

The NTSB concludes that the captain’s decision to perform the engine fire checklist was appropriate given his training, the information provided by ATC, and the fire warnings in the cockpit. However, the design of the engine fire checklist delayed initiating the evacuation checklist, shutting down the left engine, and commanding an evacuation from the cockpit.

With regards to the 30 seconds wait after discharging one bottle of fire agent the NTSB analysed:

A Boeing fire specialist stated that the 30-second wait period in between discharging halon bottles was necessary because it would allow pilots time to deploy a secondary means to suppress an engine fire if it reignited due to hotspots within the engine core. The fire specialist also stated that the wait period would likely not be required on the ground; however, that information was not included in the checklist.

With respect to the evacuation the NTSB analysed in part:

Although the actions of flight attendant 7 in assessing and opening the left overwing exit did not slow the evacuation at this exit, performing flight attendant 6’s assigned responsibilities was contrary to company procedures and training. Also, flight attendant 7 deviated from the evacuation procedures in the Flight Service Inflight Manual, which indicated that “prior to opening an exit, assess conditions outside to determine if exit and escape route [are] safe,” and began evacuating passengers from the left overwing exit. However, after opening the left overwing exit, flight attendant 7 should have recognized, from the sound of the engine, that the exit would not be viable for an evacuation. (The sound of the engine would have been the primary cue to flight attendant 7 that the engine was still operating.) Given the location of the left overwing exit relative to the left engine, flight attendant 7 should have blocked the exit until the engine was shut down.

Flight attendant 7 stated that he assumed that the left engine was still running but not at “full blast mode” because the airplane had come to a stop. He also stated that his main concern was getting passengers off the airplane because of the fire. However, the evacuation guidance specifically indicated that “engine(s) still operating” was an unsafe condition. The one serious injury that resulted during the evacuation occurred after a passenger evacuated using the left overwing exit. Once on the ground, the passenger stood up to get away from the airplane but was knocked down by the jet blast coming from the left engine.

The NTSB concludes that the flight attendants made a good decision to begin the evacuation given the fire on the right side of the airplane and the smoke in the cabin, but the left overwing exit should have been blocked while the left engine was still operating because of the increased risk of injury to passengers who evacuated from that exit.

With respect to carry on luggage carried during evacuation the NTSB analysed:

Video taken during the evacuation and postaccident interviews with flight attendants indicated that some passengers evacuated from all three usable exits with carry-on baggage. In one case, a flight attendant tried to take a bag away from a passenger who did not follow the instruction to evacuate without baggage, but the flight attendant realized that the struggle over the bag was prolonging the evacuation and allowed the passenger to take the bag. In another case, a passenger came to the left overwing exit with a bag and evacuated with it despite being instructed to leave the bag behind.

Passengers evacuating airplanes with carry-on baggage has been a recurring concern. ...

Although the FAA took action in response to Safety Recommendation A-00-88 to preclude passengers from evacuating with carry-on baggage and provided further related guidance in ACs 121-24C and 121-29B, the NTSB concludes that evidence of passengers retrieving carry-on baggage during this and other recent emergency evacuations demonstrates that previous FAA actions to mitigate this potential safety hazard have not been effective. ...

The NTSB further issued extensive analysis to communication and interphone issues, recurring evacuation issues as well as the failure of the stage 2 turbine disc.
Aircraft Registration Data
Registration mark
N821NN
Country of Registration
United States
Date of Registration
Bb mnclpnAjj jkA Subscribe to unlock
Manufacturer
BOEING
Aircraft Model / Type
737-823
Number of Seats
Jkq Subscribe to unlock
ICAO Aircraft Type
B763
Year of Manufacture
P ig Subscribe to unlock
Serial Number
Nlepn Subscribe to unlock
Aircraft Address / Mode S Code (HEX)
m nic Subscribe to unlock
Engine Count
C Subscribe to unlock
Engine Manufacturer
JbnbA lq Subscribe to unlock
Engine Model
DAqcfpejikpf Subscribe to unlock
Engine Type
BibAlilek Subscribe to unlock
Pounds of Thrust
Qcmqq Subscribe to unlock
Main Owner
CcAji Agg AmbhkicqpdjfAkhidgdnjfd bjjihdkgecjpfjkmj jldjegAimg dfjbldebkqmdgincdlfqcgmmqllcnnqkkAbcfngfkeicib kA Subscribe to unlock
Incident Facts

Date of incident
Oct 28, 2016

Classification
Incident

Cause
Rejected takeoff

Airline
American Airlines

Aircraft Registration
N821NN

Aircraft Type
Boeing 767-300

ICAO Type Designator
B763

This article is published under license from Avherald.com. © of text by Avherald.com.
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