Preamble
On February 20, 2021, a Boeing 777-200 from United Airlines — on a scheduled U.S. domestic passenger flight from Denver (DEN/KDEN) to Honolulu (HNL/PHNL) — suffered a catastrophic engine failure just four minutes after takeoff.
The flight UA-328 was in the initial climb out of KDEN RWY 25 when the right hand engine's (PW4077) inlet separated associated with the failure of the engine.
The crew declared Mayday reporting an engine failure. The aircraft stopped the climb at about 13,000 feet, returning to KDEN for a safe landing on RWY 26 about 24 min after departure.
With 229 passengers and 10 crew, there were no reported injuries to persons onboard or on the ground (debris impacted the neighborhood of Broomfield, CO — located about 16 NM west of Denver — near 13th and Elmwood Street, damaging a number of homes, though).
The National Transportation Safety Board (NTSB) is investigating the incident. Similar 777-200 series aircraft have been grounded by several national aviation authorities, including the FAA, which issued an Emergency Airworthiness Directive for Pratt & Whitney PW4000-112 series engines. As of July 2021, United Airlines, which also had a similar incident in 2018, had not returned its P&W-equipped Boeing 777-200s into service yet.
From a Flight Operations Engineering perspective, although unrelated to the incident itself, it is interesting to think about some WHAT IF's here:
WHAT IF the failure happened earlier/later? Would that had any impact in the procedure adopted by the crew?
WHAT IF the environment conditions were more severe?
WHAT IF the payload was higher?
In resume, what would be the consequences of such engine failure in a worst case scenario? Let's check it!
The Departure
First, let's check where all this happened: based on the incident report and FlightAware data, it is possible to verify that the flight was authorized to departure via SID ZIMMR 2 (RNAV):
FAA terminal chart is reproduced here as well, for reference on what the crew had in hands to follow:
We can conclude from above that the failure happened somewhere between MUGBE and RALFI, where debris were found and the crew made contact with ATC prior to deviate from the planned SID and return to KDEN performing a left turn when the altitude was around 13,400 ft.
The Terrain
With the flight path in mind, let's then take a look at the terrain around the airport and the route itself.
Denver, elevation 5,434 ft, is situated on the high, flat plains along the eastern edge of the Rocky Mountains. The foothills of the Front Range of the Rocky Mountains is 26 NM to the west, rising sharply in to just over 9,000 ft at 30 NM west and up to 13,500ft around the enroute fix ZIMMR.
The following image shows the terrain vertical profile (values are non-conservative and are just for illustration, as it doesn't consider the required lateral and vertical margins):
For our analysis, we shall focus then in the segment between MUGBE and ZIMMR, as it is the critical one for the SID (obstacles after it are "shadowed"). As terrain just start rising over 25 NM from the aerodrome, we can expect that no critical limitation is expected for commercial jets, but let's proceed with our study anyway, as some good lessons can be obtained from it!
The Analysis
To start our aircraft performance analysis, let's check the SID requirements for the all-engine climb out (highlighted in the FAA terminal chart):
Reach MUGBE at or below 10,000 ft, RALFI at or above 12,000 ft, RAPDS at or above 14,000 ft, PCKNS at or above 16,000 ft, and ZIMMR at or above 17,000 ft
Minimum climb gradient after MUGBE is 3.78% (230' per NM) until reaching 15,400 ft
Speed should be 250 KIAS or greater after reaching 10,000 ft (which is actually expected for commercial jets, as standard climb schedules consider 250 KIAS up to 10,000 ft, followed by acceleration to a higher KIAS to be kept until a later transition to a constant Mach until reaching Top of Climb)
We can derive from it that, considering we reach MUGBE at 10,000 ft (as flight UA-328 did), and then keep the minimum all-engine climb gradient required of 3.78%, we would actually cover all the takeoff minimums indicated in the chart.
Going further with our investigation, I would propose the following steps:
A first step should be to check if the aircraft is able to keep the SID gradient even in case of an engine failure.
In case this gradient cannot be met with one engine-out, we should then look for the minimum gradient for obstacle clearance only.
Lastly, in case none of the above can be met, then a decision point within the SID must be defined, which will indicate from where the aircraft cannot proceed the SID and must then follow a diversion (i.e. an Engine-Out SID, or simply EOSID).
Note: A weight limitation could also be imposed to increase climb gradient, but we are here to optimize operations, right!
Assumptions
With this plan in mind, we need now to define our worst case scenario to be used for the calculations. Based on statistical weather data, I had come to the following (realistic) condition:
A rainy day during hot summer (30 ºC), with 10 kt wind from east
Maximum payload (considering a non-pandemic high season, during summer break)
For the baseline B777-200, we would then have the following parameters to be used for all-engine climb gradient and climb out path:
Actual Takeoff Weight (ATOW) = 240,000 kg
Climb schedule = 250 KIAS
Pressure Altitude = 10,000 ft
Temperature = ISA + 25 ºC
Wind = 10 kt (tailwind)
Calculation
The data we need to make our calculations are provided by manufacturers in tables and/or graphs within their Operations Manuals, or digitally via a calculation software. In the case of Boeing, that is provided through their software Boeing BPS (Boeing Performance Software) or PET (Performance Engineers Tool) for newer aircraft models, as well as part of their OPT app for EFBs (only the all-engine climb gradient, though).
For more detailed analysis and to design Engine-Out SIDs, then BCOP (Boeing Climb Out Program) must be used. Moreover, in our study, it is the perfect tool as it has a specific Gradient Report tool which can be used to check both all engine and engine inoperative climb capability after takeoff.
First, we should check the all-engine climb gradient to have a feeling of the situation, which is given below:
Given the distance from the start of takeoff until reaching MUGBE, we can see that AEO climb gradient would be much greater than the minimum required of 3.78%.
In the case of the engine-failure, though, the situation changes a bit, as this minimum gradient can't be kept, being less than 3.78%, as could be checked using BCOP.
In that sense, we are left with the need to check minimum gradient necessary for obstacle-clearance (which, from our terrain analysis, and considering necessary margins, would be 2.03%). If that cannot be complied with, then an Engine-out SID would be required.
Considering the worst case scenario we got an engine failure climb gradient of over 2.5%, with which we conclude that no Engine-out SID would be required.
Conclusions
It is part of Flight Operations Engineers' duty to not only consider engine-out scenario during takeoff run (at V1), but also during climb (after V1). Moreover, it is also their responsibility to provide crew with tools to calculate/check compliance with all-engine climb gradient of SIDs.
This incident in Denver exemplifies that responsibility, even though it was far from be a critical scenario, as obstacles are not so close to the aerodrome as we could find in other aerodromes, such as Kathmandu and Tegucigalpa, just to give a few examples.
Our analysis shows that crew was under control of the situation and, gladly, made the aircraft to return safely to Denver. But let's remember: it was not a matter of luck, but due to a great job done in the cockpit and in the back offices of United Airlines (OCC and Flight Ops Engineering), and certainly in collaboration with ATC and cabin crew during the emergency.
As always, we hope that the lessons-learned from this incident bring us toward an even safer aviation environment! Safe flights!
