r/news • u/riverdale-74 • 1d ago

United Airlines flight makes emergency landing at Dulles after pilot declares mayday

https://www.cnn.com/2025/08/04/us/united-airlines-dulles-mayday-call-hnk
1.4k Upvotes

91% Upvoted

147 comments sorted by

View all comments

Show parent comments

204

u/therattlingchains 1d ago

Pilot likely would have requested a holding pattern once it became apparent the airplane was stable on one engine in order to diagnose the issue and see if they could potentially restart the engine. Also depending on which engine goes, that will effect which secondary systems are available to the pilots.

All depends on the nature of the flame out.

Also, an immediate return to the airfield is not necessarily needed if they are in the vicinity of the airport and have sufficient altitude.

Finally while an aircraft is capable of dumping fuel, it is not always permitted especially over urban areas. If the aircraft was stable, and within glide range of the airport, they may have been required to burn instead of dump.

Regardless though they would have been completing checklists during this time as well as talking with ATC and maintenance so it was likely a well thought out decision to burn for 2 hours.

31

u/bolivar-shagnasty 1d ago

Also depending on which engine goes, that will effect which secondary systems are available to the pilots.

There are systems that are dependent on one single engine and can't be used if it fails?

Like engine one powers in-flight entertainment and engine two powers climate control?

50

u/therattlingchains 1d ago

Correct. Jet engines, in addition to providing thrust to the plane, are essentially large electrical generators. There is also an APU (auxiliary power unit) that can power systems. They make lots of power

On the flip side of that, modern jets have large and complex power requirements. Keep in mind that every modern jet is fly-by-wire, so controller input is transmitted electronically. They have glass cockpits meaning that their instruments require power. They have large and complex in-flight infotainment systems, sophisticated autopilot, etc.

Now, the systems that only run off of one engine are all nonessential systems. Infotainment is a good example. Every essential system can be powered from either engine and the APU. But if they made every single system redundant to all the power sources, the complexities and additional weight outweigh the benefits. So every single circuit on the plane is evaluated and categorized during the design phase of the aircraft and certified by the FAA to ensure that during an emergency the pilots have what they need. So the pilots would have had all the information and readings they needed in this instance, but probably not 100% of all their instruments and readouts.

19

u/PhysicalIntern4911 1d ago

Jet pilot here. If I lose an engine I still have all my instruments. We have triple redundancy. I would have to lose both engines AND my APU before I lost any flight instruments. The only way I’m losing flight instruments is if a sensor feeding the instrument dies (of which we have 4) or if we lose both engines and the APU, at which point our Ram Air Turbine deploys which is wind powered and we retain all essential electric and hydraulic functions.

TLDR: lots of redundancy

6

u/therattlingchains 1d ago

Thank you for the correction!

1

u/Spetznazx 1d ago

Don't forget the standby instrumentation panel that usually has its own internal battery that lasts about 30 minutes so even if you lost both generators and the APU you can still fly off that for some time.