Why the pilot taxi the aircraft with only one engine??

 

If you are travelling then you see in some aircrafts like bombardier q-400, a320, b737. 

there is sometimes only one engine is running and other is shut down during taxi don't worry there is no problem with the engine. For this situation there three major reasons. 

** Reason no-3= 

The aircraft engine noise is probably from 80db to 140db it will create a major impact on person which is working on ground like us. if we do not using the headset then it can lead to a hearing loss also. As a passenger you are inside the aircraft that's why that's why you are not getting the noise too much. 

If we shut down the one off those engine we can reduce the noise to more extent.

** Reason no-2= Engine Wear

Engine wear in the case of the taxi operation is minor but there are some procedure to be followed to shut down the engine after landing or during taxi in case of some aircrafts.

but the a lot of time the maintenance report states that lot of engine failure and damage takes place during taxi operation because of foreign object damage like-

This can damage the engines of aircrafts too badly believe me- 

**Reason no-3=  fuel saving  is the primary reason.

One b737 burn a fuel upto 300kg/hr at idle power. 

For 60min= 300kg.

      60min=  150kg( with one engine)

      30min=. 75kg of fuel saving.

Ok let take the example of Allaskan airline they have 160 b737 in there fleet.

Ok, now imagine each aircraft has taxi Time of 15min then with one engine they save 37.5kg of fuel. Ok now they have 160 planes then.

37.5kg× 160= 6 tonnes

Now imagine each aircraft perform a 2 flights pet day then,

6 tonnes× 2= 12 tonnes of fuel saving in a day.

Now, 12 tonnes× 365 days= 4380 tonnes 

Ok, 4380× 733 dollar( aviation fuel price)= 3.2 million dollars that's very huge amount right.

Ok, but how it is done we can very well known that our aircrafts can very well taxi with on engine and it can supply the power to the all airplane systems.

If you notice the above picture the b747-4 or b747-8 always shut down the no-3 engine after landing. 

Ok if we look at the hydraulic systems of the b747 then we realize that, engine hydraulic system are pressurised by engine driven pump.

By the no-1 engine hydraulic system we can steer the aircraft nose gear and main body gear of aircraft.

And by no-2 engine hydraulic system we can activate or pressurise the break system of all landing gears. This two hydraulic system plays the major role during taxi.

Or by engine no-4  Hydraulic system there is function of normal Break system. So shutting down of engine no-4 is the last option.

Ok, if you see the engine no-3  there is nothing special useful system is required so we can shut down the engine no-3 during taxi operations of b747 aircrafts.

If we shut down any engine for ex engine no-3 the system two called engine demand hydraulic system still working after shut down the engine.

but in most of the airlines they prefer to start the all engines for engine warm up and for see the engines performance or to check any hydraulic leakage after starting. This will be check by ground staff after engine starting and notify the pilot if there is any fire or any leakage. Like this-