Attention planespotters

April 7, 2006 4:06 PM

I snapped this picture of a plane on a typical approach for landing at Dallas-Fort Worth airport. Upon closer inspection, I noticed a small propellor deployed under the plane. Googling taught me its a ram air turbine that is used for emergency power during a hydraulic/electrical failure. Does anybody know if its used under normal conditions, or did a happen to snap a picture of a plane during an (unpublicized) emergency landing?
posted by punkfloyd to grab bag (17 comments total)

The device certainly doesn't need to be used under normal operating conditions, but it wasn't necessarily an emergency. The pressure in the plane's hydraulics may have been a bit low and they used the device to help compensate. Just one possible scenario.
posted by pmbuko at 4:10 PM on April 7, 2006

It could be a Ram Air Turbine. A RAT is a propeller driven hydraulic pump tucked under the belly of some large planes. The RAT can supply just enough hydraulic pressure to land a plane.
posted by spork at 4:18 PM on April 7, 2006

never mind
posted by spork at 4:20 PM on April 7, 2006

If you missed it, read about the Gimli Glider for interesting stuff involving this device.

I'm just speculating, but if I had a ram air turbine, I would test it from time to time.
posted by TheOnlyCoolTim at 4:28 PM on April 7, 2006

That's a Boeing 777. I've no idea how common it is, but here's another photo of one with it out.
posted by cillit bang at 5:08 PM on April 7, 2006

wow. i dunno, but that's hella-cool (i read the gimli glider stuff last week when it was on the blue (iirc)). i find it hard to believe that hydraulics can be "a bit low" in normal use - pmbuko are you guessing or do is that from actual experience? is there a public register of aircaft accidents and emergencies anywhere?
posted by andrew cooke at 5:10 PM on April 7, 2006

faa incidents for last 10 days.
posted by andrew cooke at 5:17 PM on April 7, 2006

incident database.
posted by andrew cooke at 5:19 PM on April 7, 2006

I've no idea how common it is...

Rather to very, esp at DFW. The 777 is American Airline's largest and longest flying jet, and AA has some 50 of them.

That's definitely the hydraulic RAT, and it is definitely not normal for it to be deployed. However, it would be deployed well before the full loss of hydraulics -- if one of the electrical pumps or engine pumps failed, I'd bet that standard procedure is to deploy the RAT, in case another pump fails during landing.

So, the plane may have been shy a pump and had the RAT deployed just in case -- it is exactly the sort of item you'd expect to find in a "landing one engine out" or "landing one hydraulic out" checklist.

The 777 has three full hydraulic systems, one driven from the left engine, one from the right, and the center driven by two mechanical pumps. I'm not certain if the RAT can pressurize any of them, or only the center.
posted by eriko at 5:28 PM on April 7, 2006

according to this site the 777 also has an apu (aux power supply) that should be used before the rat (if i understand correctly). see, for example, here.
posted by andrew cooke at 5:39 PM on April 7, 2006

The APU is in the tail (you can see the exhaust port from the left side of the airplane) -- it's basically a tiny jet engine that spins a generator and a compressor.

It's almost always running during takeoff and landing (and while the plane is parked -- it can also provide enough HP air to start the engines.)
posted by eriko at 5:55 PM on April 7, 2006

do you'd need the apu and the rat if an engine or hydraulic system had failed? or is the rat there as a backup for the backup (apu) for the remaining hydraulic system?
posted by andrew cooke at 7:20 PM on April 7, 2006

That database indicates a MD-88 hydraulic failure at Dallas-Fort Worth. But it seems to be the wrong airplane.
posted by Ironmouth at 11:16 PM on April 7, 2006

I just wanted to put my vote in for eriko's comment. That is indeed an AA Boeing 777, and that is indeed the Ram Air Turbine.

Do you have the original (or larger sized) photo anywhere? I'd like to see if I can get the tail number off of it.
posted by drstein at 11:42 PM on April 7, 2006

drstein: The tail number is N79IAN (or N791AN). Thanks for all the insightful responses!
posted by punkfloyd at 6:11 AM on April 8, 2006

N791AN.

Most of the RAT photos on airliners.net say they're from test flights, and since American is headquartered in Fort Worth, it would make sense for them to be doing one there.
posted by cillit bang at 6:23 AM on April 8, 2006

is the rat there as a backup for the backup (apu) for the remaining hydraulic system?

The RAT is the last gasp backup -- if you have no engines and no APU, the RAT will provide enough power to run the critical air surfaces and the critical navigation and radios. If you've lost engines, APU, and the RAT, you fall out of the sky.¹ The batteries would let you tell everyone you were falling out of the sky, but they wouldn't be able to spin the hydraulic pumps enough to save you.

The APU, in domestic flights, is basically used to provide power when the engines aren't running, and to provide the compressed air needed to start the engines. It also acts as a backup generator, in case of an engine failure (or a generator failure on one of the engines.) On overseas flights, esp when ETOPS² is in play, they're a mandated item, in case you lose a generator or engine while over water.

It's perfectly possible to fly without the APU -- you need to hook up to ground power on the ground, and you'll need a start cart to start the engines. Domestically, you can often fly legally with a dead engine generator if you have a functioning APU and batteries. The term here is MEL, Minimum Equipment List, which shows what needs to work to be able to legally fly and carry pax. Ideally, everything works, but if you lose a generator away from base, but you have a functioing APU and batteries, you can fly home legally. The MELs get complicated, but to the flight crews and mechanics, it's a check list. They look up the broken item on the MEL, if it is there, it'll either list what you need to have if it's inop, or it won't, and if it doesn't, you fix it or you don't fly.

[1] The Gimli Glider lost engines and APU for the same reason -- no fuel to run the engines, no fuel to run the APU. The RAT worked, and the rest is history. The plane still flies in Air Canada service today.

[2] ETOPS - Engines Turn Or Passengers Swim Extended Twin OPerationsS. Before ETOPS, a twinjet had to remain less that 60 minutes flight time away from a suitable airport, flying on one engine. In the 60s, this made cross country flights hard, and internationals impossible. This also explains the tri-jets. ETOPS increases that time. ETOPS-120 means you can be two hours away, which means most of the Atlantic routes and quite a few of the Pacific are available, thanks to places like Gander and Iceland. ETOPS-180 puts most of the Pacific routes in play, the newest is ETOPS-205, which basically puts anywhere in play, with the exception of Antartica.

ETOPS is also why we still have the 747 and A340, since four engine craft aren't restricted. There's also a size factor. A twin jet has to be able to fly on one engine, while a quad jet has to be able to fly on only three engines. This means that twinjets are normally quite overpowered -- the 757 famously so, the fastest climbing passenger jet in the sky. A single engine that could fly a fully loaded 747-400 doesn't exist (but it's close, the GE-115 could do it with some weight penalty.) There flat out isn't one that could keep an A380 in the sky alone.

There are engines that can keep an A340 in the sky with just one turning, which is why Airbus also sells the A330, which is the same plane as a twinjet. But the A340 is considerably smaller than the 747 and A380.
posted by eriko at 10:10 AM on April 8, 2006