Incident
On an early morning flight, WestJet Airlines flight WJA 28, a Boeing 737-200 (serial number 21770), departed Abbotsford International Airport, British Columbia, at 0632 Pacific standard time, destined for Calgary, Alberta. The aircraft carried 5 crew members and 52 passengers. Shortly after passing through 1500 feet above ground level, the pilots heard a series of loud bangs, similar to an engine compressor stall. Engine instruments indicated abnormally high exhaust gas temperature and declining power turbine speed on the No. 2 engine. The pilot flying leveled the aircraft at approximately 3000 feet above ground level and executed the engine limit/surge/stall quick reference checklist. After confirming a problem with the No. 2 engine, thrust was reduced to idle, at which point the engine appeared to operate normally. Flight attendants reported no visible damage to the engine. The pilots notified Abbotsford Tower of their intention to return and informed the passengers. Airport emergency response services were called but not required. The aircraft landed on Runway 25 at 0718 and taxied back to the terminal without further incident. No injuries or fire occurred.
During the initial climb, engine performance and cockpit indications had been normal until the loud bangs. Passengers seated at the rear right side observed flames emanating from the engine exhaust in conjunction with the bangs. The captain advised passengers that a technical problem with the No. 2 engine had occurred and they were returning to Abbotsford.
Investigation
The Transportation Safety Board (TSB) Engineering Branch conducted an investigation, issuing report LP 08/02 – Nose Dome Detachment. The investigation focused on the nose dome assembly and front accessory support. Examination revealed that the coarse thread of stud number one was significantly damaged prior to installation in the thread of stud boss number one. Installation of that stud damaged the thread of stud boss number one. Eventually, the remaining thread material in stud boss number one could no longer maintain the required hold-down stress, failed in shear, and began to work in the stud boss, polishing the bore.
The loss of clamping force at stud number one led to a disproportionate load on studs two and four. The stud boss threads holding studs two and four subsequently weakened and failed in shear, again followed by working in the bore and generating rub damage. Finally, the total hold-down clamping force from studs one, two, and four became insufficient to resist applied loads. The nose dome levered stud number three out of the front accessory support, bending stud three and breaking off stud boss number three.
The damage to stud number one was most likely caused by rotational engagement with a harder material. The front accessory support is made of cast magnesium alloy, significantly softer than the stud material, and contains no threaded inserts. Therefore, stud number one could not have been damaged during installation. Initial suspicion of damage from previous installation in a helicoil-repaired stud boss was dismissed due to the symmetrical nature and large number of affected thread crests. Possible scenarios include rotation of the stud while held in a vise or rotation of locking pliers around the threads.
Other operators of Boeing 737-200 aircraft that have experienced worked or damaged threads in accessory supports have implemented repair schemes to strengthen stud bosses by installing stainless steel inserts. Had the incident accessory support incorporated strengthened stud bosses, the nose dome might not have detached, even with one damaged stud.
The loud bangs and engine indications were consistent with an engine compressor stall, created by the dislodged dome disrupting airflow into the engine, rendering it incapable of producing full thrust. Engine tear down confirmed no signs of fire. The flames seen by passengers were likely from engine surges due to distortion of the inlet flow field.
Findings
The TSB determined that the engine suffered a series of compressor stalls resulting from the in-flight detachment of the inlet nose dome assembly. The nose dome detached from the front accessory support due to loss of hold-down clamping force, and the threads on stud number one were damaged before installation in stud boss number one.
Aftermath
WestJet Airlines initiated a fleet-wide campaign to replace all engine accessory supports (part number 633759) with modified supports featuring strengthened stud bosses. The modified accessory supports have threaded stud bosses reworked and stainless steel inserts installed in accordance with MTU Maintenance Canada technical order 72-21-01-01. Transport Canada communicated with the Federal Aviation Administration about a possible Airworthiness Directive to mandate replacement of all such accessory supports. The TSB released the investigation report on 06 May 2003.