Any pilots out there? I have a question.

[Gen Stonewall]

Every time I go to the airport, I see prop planes taxi with only one engine running. The second they exit the runway after landing, the pilot will turn off an engine. As for jet planes, whenever I see a jet parked, one engine's fan blades will rotate slowly while the other engine's blades will be stopped. Why is this so?

 

[Smaulz]

save fuel

 

[notfred]

Never seen that before. At our small airport, twins would always taxi w/ both engines running.

 

[Tominator]

Not a pilot but I'd guess they only need one to taxi and it keeps electrical power going.

 

[Zuph]

Don't planes have to have an engine on to generate power? I'd think it'd be a waste of battery power to taxi a fully loaded 737 or other large plane. They prolly do it so they don't waste battery power, if they even HAVE enough battery power to move without an engine on.

 

[db]

Like Smaulz said, it's to save fuel.
ie, average taxi burn for a DC9 is 400 pounds (almost 60 gallons).

 

[Gen Stonewall]

Originally posted by: db
Like Smaulz said, it's to save fuel.
ie, average taxi burn for a DC9 is 400 pounds (almost 60 gallons).

Thanks- I didn't know that fuel loss was quite that much.

 

[rudder]

Originally posted by: Stonewall
Every time I go to the airport, I see prop planes taxi with only one engine running. The second they exit the runway after landing, the pilot will turn off an engine. As for jet planes, whenever I see a jet parked, one engine's fan blades will rotate slowly while the other engine's blades will be stopped. Why is this so?

As the others have said to save fuel primarily.

If a plane is parked the fans blades are probably rotating due to the wind. Unless there is a mechanical item to check out, the engines are shut down as quickly as possible. The fans blades are not geared to the engine in any manner so they will just turn with the wind.

Planes use the engines to generate electricity and air conditioning, but on the ground they can utilize and auxiliary power unit (a small jet engine tucked inside the fuselage) or they plug into airport power.

 

[aeroguy]

damn you rudder, you beat me to the punch just when i thought my aerospace engineering degree and internship at american airlines would pay off.

Don't planes have to have an engine on to generate power? I'd think it'd be a waste of battery power to taxi a fully loaded 737 or other large plane. They prolly do it so they don't waste battery power, if they even HAVE enough battery power to move without an engine on.

no, they can't move without an engine on, there is no power supplied to the wheels. all thrust is supplied by the engines

 

[Gen Stonewall]

I have another question: What mechanism makes the turbofan blades spin? I would assume the electrical system would do the work.

 

[ScottMac]

The exhaust gasses from combustion are channeled through the compressor blades (looks like a disk with deep spiral grooves or tight fan blades...actually a couple of 'em).

To start the engines, compressed air is fed to the chamber to get the blades spinning. When they reach a certain RPM, then the igniters are lit, fuel is fed, and combustion keeps 'em spinning.

Actually, the compressors are in the front, I'm not sure if the rear blades are also called compressors or not, but I believe so.

FWIW

Scott

 

[PsychoAndy]

Originally posted by: db
Like Smaulz said, it's to save fuel.
ie, average taxi burn for a DC9 is 400 pounds (almost 60 gallons).

i remember an issue of pop mechanics or something had something interesting.

theres actually a machine that latches onto the front wheel of an airliner and is able to tow a fully loaded 747. apparently, its easier to maneuver since you can get a better view of things from the ground, and the fuel savings are pretty hefty.

 

[crab]

Some turboprops don't have an APU, one engine has a brake/clutch that makes the prop stop, but the turbine keeps running.

Main reason they shut one down is fuel, but another is so the prop isn't spinning. Most turboprops are too small for jetways, so you exit/enter right on the ramp. Decapitating PAX would suck, plus the couple minutes saved not waiting for the prop means faster turnarounds.

 

[notfred]

Originally posted by: Stonewall
I have another question: What mechanism makes the turbofan blades spin? I would assume the electrical system would do the work.

learn how a jet engine works.