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Comment by mrpippy | Hacker News Reader

I don't think there are diversion points, you either keep going to destination or turn around. The A380 is rated for ETOPS-330, that's 5hr30min from a diversion airport.

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Incase anyone is wondering about ETOPS-N

For example, if an aircraft is rated for ETOPS-180, it means that it is able to fly with full load and just one engine for three hours. [1]

Obviously in this case it 5hours 30 minutes on one engine at full load.

-- Slight edit: Unclear if with a 4 engine its rated with 2 functional or still 1 functional engine.

[1] https://en.wikipedia.org/wiki/ETOPS#Usage

A380 has 4 engines, so maybe it doees this with more than one ?

I think it means it can do it with 1, but the fact it has 4 gives it great redundancy.

I'm not sure. In the case of 4 engines, it may be 2 is how they certify it. Specifically I think the case where both engines on the same wing fail (as the worst case other that losing 3)

The rating is taking into consideration that it has 4 engines. The term is just confusing because it used to only apply to twin engine aircraft.

does aircraft only operate engine as minimal as possible to save fuel or they burn more if they use fewer engine to having engine work extra because of its weight ?

Yes they’ll use more fuel than running on all engines. They always load the extra fuel that would be required for the maximum flight time with an engine out.

The extra fuel burn is due to the drag from pushing a non-working engine through the air, and from the rudder deflection to counteract the unequal thrust. It’s less of an issue on a four engine aircraft with a single engine out as they can increase thrust on the remaining engine on the side with the engine out.

Extra fuel burn is also required because a twin engine aircraft with an engine out can’t maintain the normal cruising altitude, and the higher you are the more efficient the engines are.

Thrust can’t be reduced much to save fuel because the speed margins at altitude are quite narrow - if they reduce thrust and therefore airspeed they’ll descend.

> It’s less of an issue on a four engine aircraft with a single engine out as they can increase thrust on the remaining engine on the side with the engine out.

Do they also decrease the opposite engine to help with this as well?

That's an option.

Aircraft with disabled flight controls have occasionally steered / maneuvered utilising variable engine thrust alone. A notable instance is UA 232 (1989), Denver Stapleton to Chicago O'Hare, which crashed on landing at Sioux City, Iowa. Despite a nearly completely disabled aircraft and a violent landing, there were 184 survivors of 296 souls, including the pilot Alfred Clair Haynes (he died in 2019, aged 87).

The aircraft, a DC-10, suffered an uncontained fan failure which severed all three hydraulic control systems, disabling virtually all flight control surfaces (elevators, ailerons, rudder), and the pilots (with assistance from a dead-heading pilot/instructor passenger) controlled both horizontal and vertical orientation using engine thrust alone.

I've heard and read numerous times that in simulations of the incident afterwards few or no pilots managed to land the plane. Haynes was an absolute master pilot.

<https://en.wikipedia.org/wiki/United_Airlines_Flight_232#Att...>

<https://en.wikipedia.org/wiki/Al_Haynes>

It is my understanding from a (no-longer-available) MIT OCW aircraft systems design video that these requirements are based on one engine failure on the aircraft, regardless of the number of engines on the aircraft.

I believe it's not just that it is able to fly with 1 engine. It's that the probability of a secondary engine failure in that time is below a certain threshold. Most twin engine planes can fly perfectly fine for basically any distance with an engine out, ETOPs provides confidence that the other one won't fail too.

Yes, for example the FAA requires a failure rate of better than 1 per 100,000 hours for ETOPS over 180 minutes.

do they test a failed engine or is it simulated? as in do AirBus and Boeing fly a plane intentionally fail an engine and then see what happens

There’s a bunch of regulations, but from looking through them quickly I think they start with a bunch of testing and analysis initially to show a predicted rate under the requirement, and afterwards they look at the real-world rate with a 12-month rolling average.

Flying multi-engine aircraft with an engine shut down is a routine training and test maneuver, so these aircraft have all been flown a lot of hours with an engine out. The tricky thing about ETOPS is ensuring very high engine reliability (so that the probability of coincidentally having two engines fail on a flight is low) and avoiding failure modes that would affect both engines at once (one of the reasons for a lot of redundancy in electrical systems).

And while technically different rules, ETOPS in practice gets attached to other requirements for transoceanic flights, so ETOPS aircraft often have additional life rafts and other equipment.

ETOPS per se makes no sense for a 4 engine aircraft (the T in the acronym is "twin-engine".) Three- or four-engine aircraft have equivalent engine-out long-range operations ratings, though.

Apparently the acronym can now be read as the blander "ExTended OPerationS", or according to the ICAO all such flights can be referred to as EDTO (Extended Diversion Time Operations", which is less fun to say out loud and loses the joke definition "Engines Turn Or People Swim")

In practice do they normally fly that far between possible landing spots? My understanding is that they try to normally stay within 2 hours of a possible landing spot

There is a runway (YWKS) on antarctica 'moderately' close to the point where the midpoint of the great circle for that flight is, I'm willing to bet it's used as a diversion point for ETOPS purposes for those flights. It couldn't handle airliners daily, but in an emergency (in summer at least) I bet it could be mobilized just fine.

edit: allegedly YWKS does have a regular A319 service from Australia.

> The A380 is rated for ETOPS-330

I thought ETOPS is for 2 engine aircraft. Are there minimum diversionary requirements for 4 engine aircraft?

Yes that changed some time ago, it applies to 4 engine aircraft as well.

You are correct. Diversion points are Perth or Durban. Nowhere else.

It's incredible a 14 hour flight can run with that level of certainty!

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