Why does a Boeing 737 have a tail pipe (exhaust)?

I noticed this while taxiing around an airport a few days ago and couldn’t figure out why 737’s seem to have an exhaust pipe at the back of the plane. I had a hard time finding a photo of the back of a 737 but here is one that kinda shows it.

It doesn’t seem that all planes have this but frankly I forgot to take a quick survey of other planes as I was tooling about the airport. The exhaust pipe seemed fairly large as if there was a sizeable engine attached to it. However, the engines on the 737 are on the wings and I can’t see the point of ducting exhaust through the plane and out the tail. Then again, I’m no aviation engineer so what do I know? Basically nothing as far as this is concerned and obviously the Boeing engineers saw fit to have this here so I’m sure it serves a necessary and useful purpose. I’m just curious as to what that purpose is.

I work for a company who’s name rhymes with Owing, although not in the airliner part.

I’d guess that it’s the exhaust for the APU (auxiliary power unit). This is a small gas turbine that provides energy for the aircraft systems when the main engines are not running. Sometimes they’re located in the area of the nose wheel gear, sometimes in the tail section.

I just confirmed on the company web site that the APU on the 737 is located in the tailcone.

Quote “An APU is a small gas turbine engine located in the tailcone of all current production aircraft. The primary job of the APU is to provide pneumatic power (for air conditioning and main engine starting) and electrical power during ground operations. Most models can also be started in-flight to be used for back-up electrical power.”

Thanks GaryM.

I had considered the APU but for two things:

A) The tail pipe is quite large in diameter. I’m pretty sure I could stick an arm or leg in there. While I have no idea how bog of an engine the APU is the exhaust pipe would seem to indicate it is larger than an engine on a semi. For no good reason whatsoever that seems awfully big to me for an APU.

B) Why don’t all planes have this? Or, if it is in the nose in some planes why haven’t I seen a big exhaust hole in the front of the plane?

Thinking on it the APU does seem to be the most likely culprit but I’m not certain.

Here’s some information on one of the three used:

Honeywell 85-129 APU

The GTCP85-129, which was first installed on the 737 in 1967, is the most powerful, reliable, and has the longest life of the 85 Series APUs.

The GTCP85-129 weighs approximately 340 lb.,produces approximately 230 air horsepower, has a Maximum Rated Rotor Speed of 41,100 rpm and a Rated EGT of 1275F. The -129 can provide pneumatic power up to 17,000 ft, has an operational envelope up to 35,000 ft, and has been rated with a 120 minute ETOPS.

I’d expect a 230 HP gas turbine to have a respectable volume of exhaust.

A. Answered satisfactorily above. Essentially, it’s a pretty big engine and turbines push through a lot of air.

B. All aircraft of significant size do have this. Anything from a Dash-8 twin turboprop to a B777 has an APU for running electirics on the ground and to provide power for engine start. The exhaust isn’t always right at the back as on the 737 it can also be out the side (still near the back) and they aren’t always that obvious. A good indication that there is an APU is that the aircraft is making noise but the engines aren’t turning.

I agree that you are probably correct that what I’m asking about is the exhaust pipe for the APU. However, I just wanted to point out that a 230 HP engine by no means needs a large exhaust. Many standard street cars beat that number (e.g. Infiniti G35 @ 260 HP ). Heck, Indy cars pump out narly 800 HP and I can’t even see the exhaust pipes on those ( like this shot of one from the rear ).

Maybe a gas turbine engine behaves differently than an internal combustion engine as regards exhaust or maybe the exhaust requirements when flying several hundred miles-per-hour are sufficiently different to require a large exhaust pipe. I don’t know but obviously it is what it is for a reason no matter how many photos of other exhaust pipes I care to look up.

Turbines suck in a lot of air. After all, the design of a turbo jet is such that the propulsion comes from the exhaust. Indy cars don’t use their exhaust for propulsion, it’s a by product. Of course, APUs aren’t there for their propulsion, but the design stems from the exhaust=power train of thought.

IIRC, the Turbine Indy Cars had rather large diameter intake and exhaust pipes.

But anyway, that’s the way it is.

That then begs the question of where the intake for the APU is (and don’t even think of saying it’s behind the Quik-E-Mart). I’m not looking for a huge opening but something on the order of the M1A1 Abrams snorkel.

Apparently the newest 737s are using this APU:

Honeywell 131-9** APU

The 131-9** APU is the newest gas turbine engine from AlliedSignal Aerospace for use in the 737 Next Generation aircraft. The 600 eshp engine provides the electrical and pneumatic requirements for ground support, main engine start, and emergency needs while weighing in at only 410 lbs. Primarily run on the ground, it is capable of being
started at an altitude of 41,000 ft.

This baby is 600 ESHP. I think that’s Estimated Shaft HP. I’m unable to find a solid answer about the intake. It looks as though there may be a grill type inlet built into the tailcone.

Pictures and description of a 747 APU inlet.


I’m just reading this and wondering about where I could get a whole shitload of bananas really cheap.

Apples and oranges. In addition to the comments above, the final exhaust gas temperature of a gas turbine (“EGT of 1275F”)is a bit higher than the final exhaust gas temperature of an IC engine mounted in a motor vehicle, which is also going to impact directly the gas exhaust volumetric flow rate.

Uhhhhmmmm. I build 737’s for a living. We spit a brand new airplane every day. Most everything above is correct but I have a couple of comments and a couple of answers.

Someone mentioned the installation of APU’s near the nose landing gear. I have become very familiar with virtually all commercial airplaned since the advent of the jet age. No commercial airplanes have the APU near the nose gear. Early 707’s mounted the APU inboard of the right hand main landing gear, it was moved to the tail in later models when the need for more air conditioning capacity was required.

The air inlet for the 737 APU is located just forward of the right hand stabilizer (the small wings at the back of an airplane). It has an air deflector to direct air into the air inlet if the APU is required while the airplane is at altitude, otherwise it is closed in flight. Upon landing, the air inlet opens (one of the many things that happen when an airplane changes from air to ground mode). The air inlet at the APU firewall is approximately 18 inches in diameter. The exhause outlet measures 14.5 inches (I have measured one).

The APU’s are actually not used in most major airports now days. When airplanes pull up to most modern terminals, before the engines are shut down, the airplane has ground power applied. APU’s are noisy and a bunch of them running at the same time would make a lot of noise. It is more like the emergency generator in your garage. Not always used but there when you need it.

From the time I recieve the customer okay to install, I can have one in and ready to go in about an hour. But when you 8 hours to do it, you take your time. That included the time to uncrate the APU, get a forklift to set it in the installation tool and place both on the APU stand. Wipe and dipe the APU bay, get QA and customer okays, slap it in, hook up fuel and electrical, sell the installation to QA, get okay to install APU bay door, hook up fire loop, and functional test the fire loop with QA. Of course, the 8 hours also includes BS’ing with anyone within earshot, going to breaks and lunch 10 minutes early, and the mandatory reading session in the boys room. I enjoyed working that bar, now I just test passenger cabin lighting.

GaryM: You may be thinking of the Ram Air Turbine, or RAT. These are deployed in cases of total engine failure and can provide emergency power to the aircraft’s systems.

When a 767 ran out of gas near Gimli Alberta, they had to deploy the RAT in order to maintain power to the systems. They glided about 80 miles to a safe landing after the flameout.

*Originally posted by racer72 *
Someone mentioned the installation of APU’s near the nose landing gear. I have become very familiar with virtually all commercial airplaned since the advent of the jet age. No commercial airplanes have the APU near the nose gear. Early 707’s mounted the APU inboard of the right hand main landing gear, it was moved to the tail in later models when the need for more air conditioning capacity was required.

OK, Main Gear. I said I didn’t work on commercial A/C!

And Sam thanks for reminding me of the Gimli Glider! A great story. It’s on our internal site.