Plane Answers: Airplane specific questions

Welcome to Gadling’s feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!

I get a lot of question about the specific details or features of airplanes. I thought I’d round up a few for today’s Plane Answers.

Kevin asks:

I fly on A319 and A320s a lot. As we are on landing approach I hear three shrill tones or whistles from the cockpit. Sometimes it is close to the runway sometimes not. What is it?

Hi Kevin,

I checked with my friend Chris, who’s an A320 pilot who has a good answer for what’s causing the noise.

He’s most likely hearing the autopilot disconnect siren. It has a two-tone chime that repeats three times. It’s loud enough to easily be heard in first class and sometimes the first couple of rows in coach. A lot of pilots will allow the AP to trim up the aircraft after all the configuration changes before disconnecting the autopilot. That is why he hears it at about 1000-1500 feet on visual days.

The other possibility is the flight attendant call tone. It wouldn’t happen close to the runway, but it’s so loud that it blocks all other noises–including master caution warnings. They’re trying to get Airbus to turn it down a bit.

I’ve often heard the altitude callouts on the MD-80 from first class. “50, 40, 30, 20, 10.” It seems if passengers can hear it in the cabin, then maybe it’s just a little too loud. Unfortunately, we don’t have any control over the volume. Thanks Chris for the A320 insight.

Keith asked a question that took a little more research:

I often fly on SAAB 340s from my home airport to a hub. Unlike a lot of folks I love flying on the 340. I enjoy being able to “feel” the plane flying. Before 9/11 the cockpit “door” was just a curtain, which was often left open by the pilots. If you sat up front you could see most of the instrument panel. I’m familiar with most cockpit instruments but this one has me stumped:

On the bottom right of the “captains” panel is a little device that looks like a toy. It’s a round sphere filled with liquid and another sphere with black and white wedges. Sometimes it spins wildly and other times it just moves lazily. I’ve never seen a pilot pay it any attention. Just what is this thing?

I talked to my friend Frank, a former Saab pilot who told me that he’s pretty sure the Saab didn’t have such a device, but he remembered that the Beech 1900 did in fact have exactly what you’re talking about.

Here’s a fuzzy picture he dug up for me:

The little ‘pinwheel’ at the bottom-right of the instrument panel is a device that’s used to manually ‘sync’ the propellors. When the propellors spin at different RPM’s you’ll hear a wow-wow-wow sound. It’s caused by a harmonic vibration and it’s rather annoying to the passengers and pilots.

It even happens on jets, too. You’ll notice it when sitting between the rear mounted engines on an MD-80 in the aft row of seats, or even a Boeing if you’re paying attention.

But some turboprops have a feature that the jets don’t: a prop sync.

As the wheel spins, it’s up to the pilots to increase or decrease one propellor’s RPM to slow the spinning pinwheel down as much as possible. This is only necessary when the auto-sync system, or Propellor Synchrophaser as it’s officially called on the Beech, is inoperative.

Ivan asks two questions:

Long time reader first time questioner, and you said the questions have been dryin up, and in so doin’, you’ve opened a pandoras box :-). Anyway, here goes:

On the right side of tail cones on Boeing’s 757s, there is a little tab, like a strake or vortex generator (don’t see why you’d need one back there), that looks like a mini-elevator. What is its purpose?

Most people probably don’t realize that the 757 and other twin-engine Boeings really have three jet engines on board. Two propel the airplane and one is a small jet turbine, identical to what’s used in a turboprop airplane such as the Jetstream 31. On the ground, it’s sole purpose is to provide electricity through a generator as well as air-conditioning for the cabin. Inflight, it’s used as a third source of electricity in case one engine fails.

The aft most portion of the fuselage on most Boeings is where the APU exhausts. Since there’s a small amount of fuel that leaks from the APU when it’s shut down, the APU has a fuel drain shroud that is designed to keep the drained APU fuel away from the exhaust. So here is a picture of the ‘fin’ on the right side of the aft fuselage that you saw:

Ivan continues:

And finally, one last question dealing with engines. At the aft end of a 757 nacelle, there appear to be two different cowling designs. Is there a difference as far as performance in the open end versus tapered versions. Look at AA’s 757 engines as opposed to say, Delta’s.

What you’re seeing is simply a different type of engine. AA bought Rolls Royce RB 211 engines for its 757 and other airlines, like TWA, United and Delta bought the Pratt & Whitney PW 2037 engines. If you’re curious, the Rolls-Royce has more thrust.

You can see the side by side difference below when looking at a former TWA 757 in AA colors and the Rolls Royce version on the right:

Because of the engine differences that require extra spare parts and training, AA has sold all of the former TWA airplanes, with most of them going to Delta.

Ben asked:

Kent – I was at the Atlanta airport today and I saw this crazy tug. It looked like the nose gear of the Delta jet was sitting ON the back of the TUG. Am I imagining things? I linked to a picture, sorry I couldn’t get closer to get a better shot. If you look closely under the tug, you can see that there is no plane tire touching the ground.

Yes, Ben. You’re right. That’s a high-speed tug designed to pull or push airplanes back without the nosewheel touching the ground. We occasionally see these tugs. The first clue that you’re going to be pushed back by one is when you feel yourself being jacked up a bit as if someone was trying to steal your wheels with you inside. (A rather rare event in an airliner.)

These tugs are commonly seen in France. It’s rather amazing just how fast they can pull a 747 around the airport. Perhaps someday we’ll be pulled to the end of th
e runway to save on the amount of fuel burned. It’s something that’s been proposed before and it sure makes sense to me.

Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he’ll use it for next Monday’s Plane Answers