For a plane like the Boeing 747, do they ever shut off the engine?
Let’s say the plane flies from Tokyo to San Francisco. But once it gets to San Francisco, often times it’ll soon fly to a different destination, say, Washington DC. So does it ever get a rest? Or is that how the life of a jet airliner goes…constantly in work?
When we first board a flight, we can already hear the whine of the engine even though the airplane won’t take off for another 30 min or even hours. Doesn’t this waste fuel?
How do they start a jet engine? For cars, we plug in a key and turn it to start the engine. How do they do that for a 747? And shutting the engine off?
I am sure someone else can help with the start-up and shut-down procedures, but in answer to another part of your question, the 747 (and many other aircraft) has a small engine in the tail section which forms part of the APU (Auxilliary Power Unit) which generates electrical power for running the aircraft’s electrical systems while it’s on the ground and the main engines aren’t running. This does use a small amount of fuel. The whine you can hear when you board (and the engines aren’t running) is a combination of the running APU and the aircraft’s air-conditioning unit.
Maxwell,
Yes, the high-bypass turbofans like the Rolls-Royce RB211 that are bolted to many 747’s are shut down (normally) as soon as the aircraft is positioned at the aerobridge/boarding gate upon arrival.
Having these engines operating while the aircraft is at the gate would be unbelievably hazardous to everyone who services the aircraft…besides, fuel is expensive.
The APU (Aux power unit) on a 747 is normally operating while the aircraft is being serviced between flights - that’s what you hear when you’re onboard without the engines operating. It powers the aircraft systems, aircon, and avionics. It’s located in the very rear of the fuselage, and the exhaust sits beneath the rudder.
Maxwell, this is a VERY basic engine start and stop operation from my type helicopter, the scenerio though can kindof be overlapped for numerous other aircraft.
APP=Auxillary Power Plant; AGB=Accessory Gearbox
-complete pre-engine-start checklist(fuel control levers-open; ignitors-continous;area-clear) -pushin respective engine start switch and hold [on my type helo the APP drove the AGB which had mounted onto it two large generators and three hydraulic pumps-the winch/engine start pump supplied hyd power to spin the engine up] watching engine rpm increase. Once the engine started to move it’s components (fuel pump and fuel boost pump, lube pump)turn as well. As the engine is increasing in rpm it’s drawing in air,compressing the air and forcing that compressed air into the combustion chamber. At 20% eng rpm move the engine speed control lever forward to the start position (which only now allows pressurized fuel into the combustion chamber). You already had compressed air flowing thru as well as the ignitors(think spark plugs) popping now adding atomized sprayed fuel and you (hopefully) have a continous controlled explosion. If your cockpit has old dial indicating type guages you don’t see the ignition but for a moment then the temp starts to climb as well will the eng speed increase. You don’t run the engine up to 100% yet, you need to monitor the temp, rpm, and fuel flow to ensure that it is functioning properly. …that is a very basic start
a basic shutdown: When all is said and done pull (move) the engine speed control lever back to the mininum governing position, monitoring temp, rpm, and fuel flow as per engine specs. When the engine meets/stabilizes at specific parameters pull(move) the engine speed control lever fully back to the ‘off’ position which closes off the fuel to that engine. The engine should now shutdown due to induced fuel starvation.
Multi engine aircraft start # 3 engine first followed by # 4, # 1, and then # 2. Twin engine aircraft start # 2, then # 1 engine. Some type of power is needed to turn the engine starter. This can be the aircraft APU or a ground APU. The engine master switch is turned on for the engine being started. That allows fuel to be sent to the engine, when the throttle is advanced. Air is sent to the starter, from the APU, which turns the starter. The starter is located on the EDGB. In this case, the engine isn’t driving the gear box, it is the gear box that is causing the engine to rotate. At an indication of engine RPM, the throttle is advanced from cut off to idle. This allows fuel to be sent to the combustion chamber. At the same time the ignition button is depressed (Located on each throttle), causing the two igniters to fire (located in the 10 and 2 o’clock combustion cans, inside the combustion chamber), until ignition occures. The ignition button is released as soon as ignition occures, indicated by a rapid increase in EGT, fuel flow and EPR. At around 35 to 40% RPM, air to the starter is cut off. The starter has a centrifugal shaft which will then disengage from the EDGB. The engine will continue to spool up until idle RPM is reached. Flames or fire in the combustion section will keep the engine running. Igniters are not needed after engine start to keep the engine running. The same procedure is used for each engine being started.
Engine shut down is done by lifting the throttle detent lever, bringing the throttle from idle to cut off. This stops fuel from going to the engine. As soon as the engine is confirmed as shutting down, then the engine master switch is turned off. The EGT is monitored to make sure combustion has stopped. That would be indicated by a decrease in EGT. If the EGT increases or there is smoke coming from the front or rear of the engine, than air is sent to the starter rotating the engine blowing out the fire in the combustion chamber.
Engine start and shutdown requires monitoring several instruments very closely. The EGT, fuel flow, oil pressure, hydraulic pressure, RPM, and EPR must be closely watched for any sign of problems. The man on the ground is also looking at specific items to make sure nothing is wrong, such as fuel, oil, hydraulic leaks, fire or smoke and other things.
Each airport also has rules as to where aircraft engines can be on or off. I have worked at San Francisco International for a long time. There jet aircraft are not allowed to taxi up to the gates on their own power. This is for several reasons, FOD, noise reduction, etc. but primarily safety. I have witnessed 3 times when a tug was late and the pilot decided to park the 747 himself with dire results, but luckily only minor damage to the aircraft and the passenger bridge and no injuries to the crew, passengers or ground staff. In one case the gate agent lifted the gate as high as he could, but the aircraft still had damage, but minor sheet metal damage unlike the wing spar and other collateral damage it would have occured otherwise. Minor sheet metal damage still can range in the tens of thousand of dollars to an aircraft. In all cases, the 747 was stopped by the collision with the gate as the pilot didn’t have the proper visibility to see actually where they were.
That APU in the tail of the 747 is very powerful and could by itself supply the poser for a small city.
That’s why I always shut down the left (door) engine down on a Citation as soon as I was on the rollout. We all knew the story about the passenger’s keys going through the engine.
the answer to your question about the engines is; they are shut down when they are at the concourse at the airport, what you are hearing is the APU running, that is the Auxsillary Power Unit (which is a small jet engine) that is used to power the electrical systems and environmental (heat & air conditioning) systems when the plane is loading & unloading passangers, also to start the engines and it’s also used when the maintenance crews are working on the plane when it’s not in service like towing it, troubleshooting problems or when they are getting it ready to be place back in service.
the Turbine engines are started by Bleed Air from the APU or a Ground Power Unit, but on Corperate Airplanes they have a Starter/Generator that starts the engines like those on a car but after the engine is started then the motor turns into a generator to power the electrical systems on the plane, then when you want to shut the engine down or turn it off then the engine is starved of fuel (the pilot turns off the fuel flow) because unlike a car the turbine engine has no ignition to run the engine the turbinr engine uses pressure to run on
Let me see, in my time i have worked on As332 Super Puma and As362 Dauphin helicopters, i worked on Boeing 737-500 and -700, Fokker 50, Challenger 604, CRJ 200 and i am currently working F-16 A and B models (Block 10 and 15) so i have started, and stoped, my share of jet engines .
Some of the descriptions here seems to be very type specific. On the Boeing we didn’t have any rule about what engine to start first. When we were doing pushbacks the pilot just called out which engine he would start first.
If an airplane is just doing a turn around at an airport it will most of the time have the APU running, which is what you have heard, as others have said, its not a good idea to have a main running while people are servicing the plane.
Planes are very different in their starting procedures. Some helicopters like those i have worked on have electrical starter motors, and once the engine is started, the starter becomes a generator … smart right !
The larger planes usually have an airdriven starter motor attached to the ADG (Accessory Drive Gearbox - which as you might have noticed have many names) this is suplied with air from the APU.
Others like the F-16 have such a big engine that it would require the airdriven starter engine and APU combo to start them, but there is not enough room in fuselage to house an APU.
So the F-16 stores some hydrullic pressure in 2 cylinders in the engine compartment, when the pilot activates the starter switch this pressure is diverted through a valve to a small hydrullic motor which spins a small turbine engine called a Jet Fuel Starter, this small turbine engine then spins the main engine and starts it … a 3 stage rocket so to speak.
To stop it again !! well the short version … cut the fuel.
You don’t need a wrench. When I was going to A&P school they showed us a training film on what FOD would do. A J-34 was installed in a test stand with 1/4" bolt suspended on a string about 10 ft in front of it. Engine was started and then runnup to takeoff power at which point the string holding the nut was cut. In real time, the engine just exploded. When pic was slowed down, you could follow the progress of the nut thru the engine by watching the damage it left behind it. I’d sure like to find a copy of that film cause it was very impressive.
There is a big difference between straight jets - such as the J-34 - and the mordern turbo fans on airliners. Anything going through the airintake on a straight will have to pass throught the core engine and will do a lot damage. On a turbo fan the centrifugal force imparted on the object by the fan blades would throw it towards the engine cassing and it will pass through the fan duct and out the exhaust and do little or no damage on the core engine, it will ofcourse damage the fan blades. When a new type of turbo fan engine is designed it tested by blowing of an entire fan blade while the engine is running and the engine has to keep running for atleast 15 minutes.
I have seen CFM 56 engines hit by birds, and i mean big seaguls not a little blackbird, and metal objects and the fan blades have taken damage but there was no damage to the core engine and the engine kept running and the aircraft was able to land safely
On the RB211 the HE igniters or located at about 4 o’clock and 8 o’clock positions. (on burners #8 and #12.). It has an annular combustor. (no cans) [:)]
Berny, what about firing a cap or shell to start the Phantom (not sure if it was Phantoms only, but I know they used them on it) I saw a picture of that, and you see a bit of smoke, then the plane is gone! Looks impressive, I dont want to know what it would do to a guys lungs!
So what was that cap and why did it smoke like that?
The J-75 and J-79 engines use can annular combustion chambers. The igniters are located in the 4 and 8 o’clock position cans, not in the 2 and 10 as I posted. Sorry for the mistake. The J-57 was also can annular and the igniters are located in the 2 and 10 o’clock cans. Any one igniter in any of the engines would be enough to light off the fuel for an engine start.
The USAF used a lot of start carts to start many aircraft. The F-4, F-100, F-105, and B-57 just to name a few. They are not needed now because most aircraft have some type of self contained starter system. The F-15 use their JFS to start the engines. The sound is a strange howling.
In the post above, it gives a good explanation of how the start carts operate. As for the smoke, we tried to be up wind so the smoke wouldn’t get to us. At times it did, and you could taste and smell the burned powder for a long time after.
Can’t forget the F-16’s, F/A-18’s & F-22’s with the JFS and even the F-20 had one, on the Fleagle the JFS is manually linked unlike the Lawn Dart is Electrically linked but the others your guess is as good as mine but I’m pretty sure the F-22’s is more like the Lawn Darts JFS
