rollercoaster

Adding to the above comments, most large transport category aircraft have electrically driven hydraulic pumps to backup or supplement the engine driven pumps. Therefore, you can obtain system "A" hydraulic pressure with either system "A" engine driven pump or system "A" electrically driven pump running.

rickair7777

I don't think the batteries on any transport aircraft can power the hydraulics...that takes a lot of juice. That is what the RAT is for. The batteries usually power critical avionics during a generator power loss...long enough for the RAT to deploy and to get the APU started.

Due to recent fuel conservation initiatives, most smaller airliners T/O and land with the APU off, unless there is a performance requirement for it. We used to have it on. Not sure about the heavies.

freezingflyboy

At my airline, shutting down the APU is part of the After Takeoff flow. It makes sense when you think about the possibility of losing one or both engines, which happens most often on takeoff when the engine is under the most stress. I know one of the first steps on the Engine Failure Inflight checklist is to start the APU, if available.

I used to think it was a good idea to have the APU on during landing too in case of a bird taking out an engine or something but I have since taken 2 birds through jet engines during 2 separate approach and landings with nary a hiccup from the engine. Granted, not a flock of geese but one bird was big enough to hit the lip and pylon and splatter the entire vertical stab with blood and guts while the remainder went through the engine (left a nice blood streak around the inside of the cowling). I'm sure having the engine at relatively low power vs. a takeoff has some effect on how well the engine takes it.

rollercoaster

Not all aircraft are created equally. Some RAT's only provide hydraulic pressure via a small hydraulic pump attached on the aft side.

However, Sully might deploy a RAT to use as a trolling motor.

File:757 ram air turbine.jpg - Wikipedia, the free encyclopedia

peewee

You better believe it. Thank y'all so much for solving the mystery. NOw, I wonder what else I don't know that I don't even know I don't know...(that's unconscious incompetence, in layman's training terms.

And sorry for the "I won't be back." That was abrupt. I mean, most forums eventually get somebody who comes barging in with a first post about...who knows what.

Thank y'all so much again. I knew I could find the answer here, and I did. Great stuff.

Now, if that old Plymouth Voyager minivan only had an APU on it, I wouldn't have hit that orange barrel when the engine quit at 60 mph in the construction zone...

peewee

I said I wasn't coming back, and that was the wrong way to say it...what I really meant was that I won't linger and be a bother. But this site is a real gold mine for things that I've wondered about for quite a while. Should I ask permission to ask before I ask...no, I chance to bother the experts one more time...

Two things:

How would Sully have set the flaps for a water landing? Like a ground landing? What's the training for that? After all, legend has it that this was the first totally successful ditching of a big plane. Something went "right," or more exactly, everything did.

Second, the port engine got ripped off of the wing during the landing. Why did that not turn the plane around in the water? It stayed straight down the river until it stopped and then floated with the current.

If that kind of stuff makes me wonder, I wonder how many other civilians may wonder. Well, probably not many. But for sure the press is not attuned enough to answer or ask those questions...

peewee

KC10 FATboy

Peewee, a windmilling engine will provide enough hydaulic power. When I start the engines of a MD-88, it only takes a few percent N1 (single digits) before the hydraulic systems show a rise to full pressure.

I'm not sure what damage the engines sustained, but if they were still spinning, they were producing hydraulic power.

rickair7777

The RAT on the CRJ is a generator with a direct feed to a hydraulic pump. Same idea, different transfer path.

Also true that a spinning engine will provide some hydraulic pressure...combined with the accumulators that should be enough for basic control. But a fly-by-wire airplane also needs electricity to power the "wire" part...does the Bus need AC for that, or will the battery suffice?

peewee

Rickair, I have spent maybe 60 hours on a CRJ (as a passenger). Nice ride. In my mind I named the CRJ per the acronym to be a "real jet," as compared to the smaller one....what's the smaller Canadair jet, ERJ? It's been a few years since I flew to work for a living.

Anyhow, I don't have a whole lot of confidence in answering the question about will the battery suffice. If I tried a guess, all on board may be in jeopardy if we hit a bunch of big birds, so don't trust my feedback at all.

Gosh, reading all of y'all's coments is so much fun! And a learning experience. Thank y'all again. Great stuff!

Toejam

peewee, I will continue your ever so slight diversion of the thread (sorry wmupilot69) and discuss the ability of aircraft to land on water. Like everyone says, windmilling engines and RAT (Remote Air Turbine) can and will power hydraulic systems, which then power flight controls in newer high speed aircraft, you need to notice that both are driven by wind. As the wind decreased, the devices provide less pressure, untill the pressure is not enough. Both windmilling engines and RATs have a lower limit of speed to provide adequate pressure to the hydraulic system, and it is close to landing speed. If you slow below this speed, trying to make a smoother landding, you might lose control of the aircraft and have hell to pay. This is what I was told happened to the 767 that ditched in the Caribbean.

YouTube - Boeing 767 Crash into sea - high-jack out of fuel

If you notice in the video, at the last minute, the left wing drops and starts the drag to the left that ends up carthweeling the whole aircraft. This aircraft was hijacked and ran out of fuel, and in their ditching they let the speed reduce until the RAT became ineffective. (if anyone knows better, tell the whole story and let me know),

The bottom line is that any water landing is very difficult and there is more luck than skill involved.