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Comment by m00dy | Hacker News Reader

How's that possible it managed to fly with only one engine ?

The other engine provides enough thrust to keep the plane airborne. The pilots (and likely other onboard systems) compensate for the asymmetric thrust with the rudder.

As scary as it looks, this is one type of event that pilots routinely train for, both to get their initial multi-engine certification, and on their airlines’ simulators.

They happen pretty frequently... Just normally they don't look as visually spectacular as this.

> The other engine provides enough thrust to keep the plane airborne.

We could hear the remaining (loud) engine as the plane flew over. Our house is less than 2 miles from where the parts landed and right under its flight path.

The boom just before that sounded like a sonic boom and shook our house perceptibly.

There's been some property damage (a smashed truck top and a big hole in a roof) but no reports of injuries to this time (18:45 Mountain time).

It’s designed to fly with only one engine. Not with great performance, but it will stay in the air. Routine procedure for twin engine aircraft.

Not only fly but climb.

Commercial aircraft are required to fly safely should one of the two engines fail at any point of the flight, including takeoff at speeds beyond v1.

For the uninformed:

V1: Speed at which the take off attempt should no longer be aborted

Vr: Speed at which the plane should be pitched up (rotated)

V2: Speed at which the plane can safely climb with one engine

If an engine fails after V1 the runway is supposed to have sufficient length that you can hit Vr, rotate, and be 35 feet off the ground and traveling at V2 before you run out of tarmac even with a single engine.

Fair enough and my memory may be fuzzy but I see quite a few references to V1 as being the minimum single engine takeoff speed.

https://skybrary.aero/bookshelf/books/493.pdf (page 2)

It's kind of poorly worded. Hitting V1 means you are committed to taking off, single engine failure or not - but you HAVE to reach V2 to safely climb on one engine.

This should happen within moments of hitting Vr, but being single-engine after V1 means you went from hitting Vr halfway down the runway to potentially doing so at the ass-end of your minimum runway length. This means you have basically no room for any pilot error or other failures, and why engine failure on take-off is routinely rehearsed.

Not sure where the original disagreement came from. I never said an aircraft should be able to take off at V1. Perhaps you took the "fly" literally which is fine but that was certainly not the intention.

I don't have any specialist knowledge here, but that seems to make sense. If you're under V1 then you can still abort takeoff. Having that be the minimum for single-engine takeoff essentially sets up the rule that if you lose an engine while you can safely abort, then you must.

... and to fly that way for quite some time with high confidence of success.

If you think about it: this aircraft was flying to Hawaii; after crossing the west coast, it would be flying almost 2,500 miles over the ocean to get there.

If you lost the engine halfway there (instead of soon after takeoff), you'd either have to turn around and fly back toward California or continue onwards; which is almost a three hour flight.

The concept is called ETOPS for Extended Twin Operations; and the Boeing 777 was the first twin-engine aircraft rated to do this (fly up to three hours for the nearest available airport).

Lesson's learned in the aviation industry are worth their weight in blood, so the engineers have learnt the hard way to plan for every reasonable eventuality.

Why do you think it has two engines in the first place?

Come on, we all know so they can go faster.

They could go faster with a single larger engine. Why do you think they went with two engines instead of a single larger engine? Redundancy.

Not clear to me that you'd design a commercial turbofan aircraft with one engine if you eliminated reliability requirements. You need to feed air to the engine and, to the degree you have wings, might as well have 2 engines.

The idea that the engines need to sit on the wings like they do is something seems important, because it's so often done, but is actually not necessary. You can put a single engine in the tail, or on top of the cabin, or on the top of the cockpit, or underneath. They go on the wings because when you need two (for redundancy) it's a good place to put them for balance, and because it makes them nicely accessible for maintenance.

Of course military aircraft have different requirements (and many have just one engine in the tail). Certainly I'm not enough of an aero/astro engineer to know if, positing a perfectly reliable engine, you'd mount just one somewhere on the fuselage.

Trijets are cool!

> it's so often done

The MD-80 had two rear engines and was a fairly popular plane. A lot of smaller jets still have rear engines, too.

Not sure what you mean? I didn't say it was always done. In fact the point I was making was that it wasn't always done. I gave my own counter-examples. It's just when people look at modern planes they always see two engines and think that's the way it has to be. It clearly isn't.

The MD-80 (and 727) were old turbojets though. Probably a lot harder to come up with a practical tailmounted design with a turbofan although I'm open to it being possible to mount one turbofan above the fuselage.

Citation needed. It's not at all obvious that a single huge engine is in any way better than two for a given speed.

> Citation needed.

Look up 'ETOPS'.

This is basic aeronautical engineering. Barring the need for redundancy, turbofans are most efficient and quieter with larger intake sizes, the main limiting factor being fan tip speed.

This is the reason that the 737 had the MCAS stuff. They increased the size of the engine that caused ground clearance problems, which required shifting them.

Note sure this is reliable source but “ In general, twin engine aircraft allow for faster speeds, and faster pickup...”[1]

1 - https://www.vikingair.com/twin-otter-information/twin-otter-...

While redundancy makes it a moot issue, if I am not mistaken, 777s already use some of the most powerful engines available. As for how fast they fly, I believe it is mostly set by the speed of sound.

Yes they use two of the most powerful engines... and just one of those is more than enough to fly the plane just fine almost as fast as they are allowed to... so why else would they have two of them if not redundancy?

Fundamentally people can look it up themselves - 'ETOPS' - the redundancy requirement is a written regulation.

I think it should be obvious that I am responding to the claim "they could go faster with a single larger engine." no-one is arguing against redundancy (in which matter, ETOPS is just one aspect.)

If they needed to go faster, they could do that with a single larger engine if they wanted to (which they may have to develop.) But they don't, because they're limited as you've explained.

> no-one is arguing against redundancy

Sn0wCoder said they reason they had two engines was to go faster and not redundancy, and m00dy thinks they can't fly without two engines.

Point taken that some people had mistaken ideas about twin-engine airplanes, but those mistakes are most straightforwardly resolved by the redundancy consideration alone, which dominates other issues.

> If they needed to go faster, they could do that with a single larger engine if they wanted to (which they may have to develop.)

Plus a significantly different airframe and perhaps engine intakes to deal with the transonic issue that I also mentioned. It is an idea that bubbles up perennially [1], but so far, the economics have ensured that it doesn't go much beyond the concept stage.

This has become rather silly, but it seques into a related issue: It seems to me that fan-blade failure incidents (or at least uncontained fan-blade failure incidents) have increased in frequency in the last few years. If so, maybe this is a sign that we are already pushing a bit too hard on the limits of current technology?

[1] https://www.boeing.com/features/2019/01/spreading-our-wings-...

If that was the reason, they'd paint it red!

Planes must be able to do this to be certified.

Well, assuming they have two+.

A C-172 isn’t gonna fly too far with an engine out.

But, the pilot should be assuming that its single engine will fail, and prepared for that eventuality when it's most likely, which is during take off.

Every flight the person sat at the controls ought to already know, before they're accelerating down the runway, what they're going to do when (not if, if you keep flying these little prop planes you've got to assume it'll happen eventually) the engine quits. What's beyond that airfield fence? A carpark? A pond? A school?

The "impossible turn" (landing where you took off) might legitimately be a valid strategy for your failure in some circumstances if you've thought it through, but if you're doing it because you had no plan you're very likely to find out why they call it that.

Amazingly, I believe planes are designed/certified to do everything (e.g. take off fully loaded) with only a single working engine.

And if they loose the remaining one, all goes south: https://www.youtube.com/watch?v=zn1a2lSk_UI&feature=emb_logo

Not if you have Sully in the cockpit [1]. They can still glide.

[1]: https://en.wikipedia.org/wiki/Chesley_Sullenberger

Not very well, though.

Well enough.

https://en.wikipedia.org/wiki/Gimli_Glider

> In 10 nautical miles (19 km; 12 mi) the aircraft lost 5,000 feet (1,500 m), giving a glide ratio of approximately 12:1 (dedicated glider planes reach ratios of 50:1 to 70:1).

> Air Canada Flight 143 came to a final stop on the ground 17 minutes after running out of fuel.

No, planes glide just fine. That plane simply did not have enough altitude, or lift considering the bank angle to recover

Their problem really was a lack of situational awareness / crew resource management.

As they did shut down their perfectly well working engine instead of the broken one. Obviously that’s not improving your situation in any way.

Not true - most planes must abort the takeoff if any engine fails below a certain speed.

Read up about ETOPS, which is what allows twin engined aircraft to fly long stretches over water.

To a first approximation, if it takes X thrust to stay aloft, the engines produce a combined total of more than 2X, and there is a speed above which the rudder produces enough aerodynamic force to maintain directional control.

If you want to get into the nitty gritty, look up multiengine airplane type certification requirements.

That is not a problem.

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