Updated: Mar 9, 2021
Graphic picture and video's have been circulating on the internet of an incident with a Boeing 777-200 which showed a partially disintegrated engine #2 nacelle and the engine on fire.
This obviously generated massive media coverage and an abundance of public opinion.
Early investigation points to a fan blade failure of the engine and cascading effects of excessive vibration, as a result of which nacelle parts were shed and landed in a populated area near the departure airport of Denver. By sheer luck, nobody was hurt.
In addition, the engine caught fire in flight, which fortunately had no detrimental effect on the safety of the structure of the aircraft. As a consequence of the excessive vibration of the damaged engine, there was likely much more damage inflicted on the powerplant itself and the pylon structure, but further investigation will reveal the details.
Thanks to a capable crew and fortunate circumstances, the aircraft landed safely back at Denver with no injuries reported both on the ground and in the aircraft.
This article attempts to produce some more in depth documentary information about the backgrounds of the incident and its airworthiness aspects:
The B777 type aircraft has been certified and produced with different engine configurations with engines from different suppliers (Pratt & Whitney, General Electric, and Rolls Royce). The event aircraft was equipped with Pratt & Whitney 4077 type engines.
See below the type certificates for more info.
Below the TCDS (Type Certificate Data Sheet) for the engine type PW 4077
Couple of factual remarks:
The Boeing 777 has an impeccable safety record and is one of the safest, if not THE safest commercial airliner design ever built.
Powerplants are complex subassemblies and type certificated in their own right.
This means that product development, design and production are managed and the ultimate responsibility of the Type Certificate holder, in this case Pratt and Whitney.
As a Type Certificate holder, Boeing carries for the safety of the complete type design and as such is a heavy stakeholder in engine type design.
The UK Civil Aviation Authority has issued a Safety and Operational Directive that prohibits the operation of Boeing 777 aircraft equipped with Pratt & Whitney 4077 engines into UK Airspace. See below
Another United Boeing 777-200 series aircraft on Februari 13, 2018 suffered a fan blade failure on the exact same engine type that was installed on a sister ship of the most recent event aircraft.
This failure was well documented and has striking similarities with the most recent event that occurred on 20-Feb-2021.
The fractured blade (in 2018) had been overhauled twice before it fractured, and had been subject to a Thermal Acoustic Imaging (TAI) inspection twice; in 2010 and in 2015.
It had accumulated 76704 flight hours and 14129 flight cycles when it failed.
Investigation of the inspection records after the event showed that there were minor indications with both TAI inspections on that particular blade but were misclassified as "paint".
Note that non visual non-destructive inspections often show indications that are caused by other factors than material deficiencies. It often takes experience to correctly classify indications.
After the February 2018 event, FAA mandated Thermo Acoustical Imaging Inspections on the fan blades by AD 2019-03-01. Downloadable below
On 24-Feb-2021 FAA issued an Emergency AD 2021-05-51 (downloadable below) pertinent to Pratt & Whitney 407x and 408x and 409x series engines.
This AD stipulates to, before further flight, inspect "certain" fan blades as per Thermal Acoustic Inspection (TAI) method as stipulated in Pratt & Whitney SB PW4G-112-A72-268 .
Implications will be that every individual fan blade's history needs to be retrieved and, when applicable by SB, needs to be removed from the engine, shipped to an approved inspection facility and tested. After successful pass, it can be reinstalled and a complete engine can be returned to service. The AD notes that it is an interim measure, so additional rule making can be expected. Most likely after the NTSB (National Transportation Safety Board) have completed the investigation and published their safety recommendations.
Compliance with the Emergency AD does not imply that the type can be operated into UK and Japanese airspace as there is a Safety Directive valid that effectively bans operation of the type.
Please note that the Emergency AD does not supersede the required actions as per AD 2019-03-01 which stipulates an initial TAI inspection on blades before accumulating 7000 engine cycles or before 6500 cycles at a flange split, whichever occurs first.
Subsequently, these inspections need to be carried out at 6500 cycles interval or 1000 cycles at flange split.
Any blade that fails inspection is to be discarded.
Below some of the NTSB investigation documents related to the February 13, 2018 event.
The captain written statement:
NTSB Powerplants group chairmans factual report:
NTSB Materials laboratory factual report:
Pratt and Whitney metallurgic final report:
Boeing Root Cause analysis
A technical view on these events can be rational and analytical, however, for aircrew getting confronted with these failures, it is not so rational and it is often underestimated, by parties not directly involved, how intense these events can be, as they have to overcome startle factors and have to deal with task saturation attempting to bring the people and aircraft back to safety.
In such events human factors become critically important and can make the difference between life or death.
Below the link to an interview conducted by Juan Browne (a Boeing 777 pilot himself and owner of the Aviation Youtube channel Blancolirio) with the captain, Chris Behham, of the B777 who suffered the same failure in Feb-2018, as it now appears, as the failure that happened on its sister ship 20-Feb-2021.
The interview is peer to peer type, casual, but very compelling and takes the viewer into the details of dilemma's, adversity and multitude of factors to contend with by the crew, in order to bring the aircraft in safely. It is very clear that the operational circumstances on these events are far more complex than a regular engine failure. Juan has more video's on the event (and other aviation subjects) in his channel Blancolirio.
The video is linked with Juan's permission.
Please view below:
NTSB issued the first update on the incident
This is not a investigative report but a progress report of the investigation; organisational aspects, involved parties etc.
Technical data are abbreviated as follows:
The failure occurred at climb out from Denver at 12500 feet altitude at an airspeed of 280 knots.
After the loud bang, the engine shut down by itself and fire warning activated.
As part of the engine fire checklist, both fire extinguisher bottles were discharged into the engine. The engine continued to burn until after landing and was finally extinguished by ground fire crew.
Most of the inlet cowl and fan cowl structural debris was recovered from the area and identified.
All four pressure relief doors in the cowls were found open. This could indicate an explosion or overpressure condition inside the engine cowl.
The "Spar valve" (main fuel supply to the engine) was found in closed position. This valve is activated by the fire handle (T-handle)
Multiple fuel-, oil-and hydraulic lines were found damaged on the engine and the gearbox was found fractured.
The fan hub was found to be intact but the N1 rotor could not be turned by hand.
One fan blade was fractured across the airfoil 7.5 inches above the base of the blade and another adjacent blade was fractured 24 inched above the base of the blade. The fracture surface was consistent with material fatigue. All fan ble=ades were removed and the blade showing fatigue was shipped to the metallurgical laboratory at Pratt & Whitney for investigation under direction of NTSB specialists.
Initial review of maintenance records show that the fractured blade underwent Thermal Acoustic Imaging (TAI) inspections in 2014 and in 2016. In 2018 [following the UA1175 incident], the inspection data from 2016 were reviewed. Records inspection is continuing.
The lack of efficacy of the fire suppression system is most likely attributable to the shedding of cowl parts which, when intact, form a fire containment area in which fire suppressant can be effective.
The fire that continued to burn was likely not fed by fuel as the main fuel supply was closed by fire handle activation and confirmed by visual inspection by the NTSB.
To be continued....