Plane Answers: Single-engine taxi, engine-out flight and ATC confusion

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!

Julie asks:

Recently on a short flight on a de Havilland Dash 8, I noticed as we were taxiing down the runway that only one of the props was spinning, and the other one appeared to be off. I have seen this on other flights with the same type of plane, and the first time I ever noticed it I almost thought they had forgotten to turn it on (which is really, really silly I know)!

I know you don’t fly this kind of aircraft, but I thought you might have an answer anyway. I can’t really ever see into a jet engine to tell if it is spinning during taxi, so I guess it is possible that this is something regular with all twin engine aircraft and I just never knew it? Thanks Kent!

HI Julie,

I used to fly the Dash 8’s little brother, the Twin Otter. Even then, when fuel was less expensive than today, we would taxi on one engine as often as possible to save fuel.

Jets do this rather frequently too, but as you said, it’s rather difficult to tell from a distance.

There are limitations, however. We can’t taxi on one engine when we’re too heavy or when we may have to climb a bit of a hill on a taxiway, or if the taxiway may be slippery due to snow or slush. Also, on some aircraft, we need to be sure that we can give a 2 to 3 minute ‘warm-up’ time for the shutdown engine before we take off. So short taxis usually are accomplished with two engines.
John asks:

On a recent A340-500 flight the right engine sounded as if it had trouble. The flight deck crew left the side light on for about two hours and then turned them on every 2 hours after that (this flight was from JNB to ZUR). I was just wondering if we really did have a problem. We have been on many A-340 flights and have not noticed any engine noise.

Can the A-340 fly on 3 engines?

It’s hard to know if there was a problem on your flight, but you can be assured that if there were any indication of an engine issue, such as high vibration, oil loss or over-temperature, the crew would have shut the engine down.

All airliners are certified to fly with at least 50% of their power lost. So on an A340, that means any two engines could have been shut down and the airplane would still fly. They might have had to descend to a lower altitude, but they could still stay in the air.

The light you saw was a wing inspection light, used mainly to see if any ice was building up on the leading edge of the wing. It’s also used, along with all the other lights on the airplane, as an enhancement to the “see and be seen” philosophy when in the vicinity of the airport while on approach.

I’ve ridden in the back of a 737 while pilots were taking their checkrides. One engine was pulled to idle throughout much of this training and I was rather surprised to discover just how difficult it was to tell from the passenger cabin when we were flying on one engine.

While it’s never routine to have to shut down an engine in-flight, an engine failure on a four-engine aircraft is far from catastrophic.

Rishi writes:

I was wondering if you could explain to me the following two questions, please. First, why is that when I listen to ATC talk to pilots that one or both of the parties speak reasonably slowly, and understandably, but then, a moment later, they talk so darn quickly and un-intelligibly?

Second, given that ATC talks really fast at times, how do you pilots ever get to understand what they say? I’m quite surprised because I hardly ever get to catch the name of the airlines when the controller speaks so quickly. Please explain how you get used to this. . .

Hi Rishi,

There are times when pilots have to “read back” instructions from ATC. Those read backs are often spoken rather quickly. Complicated instructions that aren’t routine in nature are spoken much more clearly. So if the phrase is something that’s expected, such as a clearance for an approach or landing, clarity often suffers.

For an example, take a listen to and click on one of the audio streams of your favorite airport. After a while, you’ll notice the controllers giving nearly the same instructions to each airplane as they clear them for the approach. It might take an understanding of instrument flying to grasp what’s being said, but you’ll get the idea.

My favorite feed is from Caracas, Venezuela. Listen to the Delta and American flights going in there try to comprehend what’s being said, in between all the Spanish that’s spoken to the other aircraft.

Often it’s not the accent that’s so difficult to understand in a foreign country, but it’s the lack of awareness of the phrases used. I remember the first time I heard a French controller rapidly say “I-call-you-back,” instead of the more common, “Standby” that I was used to.

“Wha..?” I asked the captain.

I’ve now heard that phrase so many times, it doesn’t matter how strong the accent may be, I know what they’re saying.

LiveATC is a great resource for anyone who’s learning to fly or who might be working on an instrument rating.

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 feature.