Auxillary Power Unit
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Auxillary Power Unit
Hi Folks
Can anybody tell me the role of the APU? Is it wise/recommeneded practice to fly when the APU isn't working. Does this depend on the type of aircraft? The one I'm most interesetd in is an Airbus 320.
Many thanks.
Can anybody tell me the role of the APU? Is it wise/recommeneded practice to fly when the APU isn't working. Does this depend on the type of aircraft? The one I'm most interesetd in is an Airbus 320.
Many thanks.
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Hi
Thanks for your reply.
I had googled Wikipedia before I submitted the question. To quote from Wikepdia
"This same function is also used as a backup in flight in case of an engine failure or hydraulic pump failure."
In light of the Wikepdia entry I thought I would post a question hoping to get some professional replies from experts who understand things far better than I ever could, me being a mere SLF member, and that these experts with their skill and patience could pass a little of their vast knowledge on to me in a courteous and helpful way.
Thanks for your reply.
I had googled Wikipedia before I submitted the question. To quote from Wikepdia
"This same function is also used as a backup in flight in case of an engine failure or hydraulic pump failure."
In light of the Wikepdia entry I thought I would post a question hoping to get some professional replies from experts who understand things far better than I ever could, me being a mere SLF member, and that these experts with their skill and patience could pass a little of their vast knowledge on to me in a courteous and helpful way.
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Talking about APU, I flew on a First Choice 757 in July 2006 to GCLP and back, on the return leg to EGKK, we were pushing and everything went. We went back onto stand for 5 minutes, then started engines up then went for London.
When we landed around 1hr and a bit later we parked on a remote stand and engineers were at that APU at 3am in the morning.
When we landed around 1hr and a bit later we parked on a remote stand and engineers were at that APU at 3am in the morning.
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i have limited knowledge but...
An APU is a generator which runs off the aircrafts fuel while the plane is on the ground, it provides power to electrical systems... most airlines now use GPU (Ground Power Unit) when they land at an airport as it saves them money (cheaper to burn red diesel (some airports have electrical power units) than it is jet fuel...).. the APU also provides power to start up the engines
aircraft do fly with faulty/inop APU's
HeathrowAirport, sounds like the APU packed in during pushback, probably returned to stand for an airstart
theres probably alot more than an APU does, but thats all i know
An APU is a generator which runs off the aircrafts fuel while the plane is on the ground, it provides power to electrical systems... most airlines now use GPU (Ground Power Unit) when they land at an airport as it saves them money (cheaper to burn red diesel (some airports have electrical power units) than it is jet fuel...).. the APU also provides power to start up the engines
aircraft do fly with faulty/inop APU's
HeathrowAirport, sounds like the APU packed in during pushback, probably returned to stand for an airstart
theres probably alot more than an APU does, but thats all i know
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As far as I am aware, and if I understand your question correctly...
Aircraft fly with faulty/nonfunctioning APU's all the time. WHen I worked on the ramp, we used to get messages all the time. All you did was ensure the aircraft had a GPU (Ground power unit) plugged in, and an AirStart Unit to fire up the engines for pushback.
I dont recall hearing a pilot complain about a faulty APU, and certainly never seen an aircraft pulled from service for such a reason.
RTG!
Aircraft fly with faulty/nonfunctioning APU's all the time. WHen I worked on the ramp, we used to get messages all the time. All you did was ensure the aircraft had a GPU (Ground power unit) plugged in, and an AirStart Unit to fire up the engines for pushback.
I dont recall hearing a pilot complain about a faulty APU, and certainly never seen an aircraft pulled from service for such a reason.
RTG!
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Maddie,
An auxilliary power unit is nothing more than a small powerplant or motor which is used on the ground to provide certain services. Generally the APU provides three things: hot air, hydraulic power, and electrical power. How it does so depends on the airplane and the APU, but these services may also be provided in other ways than using the APU.
Operating without an APU is generally not a big deal, if ground services are available to provide pressurized air and electricity to the aircraft. APU's are seldom used in flight, and many installations cannot be used in flight. Generally they are not a backup to aircraft systems in flight...other equipment fills that role.
On the ground, the APU helps provide electricity to open or close cargo doors, turn on lights, run various electrical items such as galley ovens or instruments, and even open and close fuel valves. The APU may be used to provide air to cool or warm the aircraft while passengers or freight is being loaded. The APU may also supply hydraulic pressure to move flaps or controls, or may supply air to turbine motors that power hydraulic pumps. These different functions can also be accomplished by GPU's, or Ground Power Units, and by "huffers," which blow air into the aircraft pneumatic system.
Aircraft are equippped with documents which spell out which items may be safely inoperative, and the conditions and limitations under which they may be inoeprarative. This document is known as the Minimum Equipment List, or the Configuration Deviation List. This document will invariably include the APU as an item that may be inoperative for a flight or series of flights. It will also clearly spell out any limitations that may apply to the flight, the length of time it may be inoperative, any mechanical work that must be done to the aircraft to make it safe (inspections, deactivation, etc), and so on. Rest assured that if you go flying with an inoperative item such as the APU, it's been very closely examined and deferred in accordance with the MEL. In this condition, it's legal and safe.
An auxilliary power unit is nothing more than a small powerplant or motor which is used on the ground to provide certain services. Generally the APU provides three things: hot air, hydraulic power, and electrical power. How it does so depends on the airplane and the APU, but these services may also be provided in other ways than using the APU.
Operating without an APU is generally not a big deal, if ground services are available to provide pressurized air and electricity to the aircraft. APU's are seldom used in flight, and many installations cannot be used in flight. Generally they are not a backup to aircraft systems in flight...other equipment fills that role.
On the ground, the APU helps provide electricity to open or close cargo doors, turn on lights, run various electrical items such as galley ovens or instruments, and even open and close fuel valves. The APU may be used to provide air to cool or warm the aircraft while passengers or freight is being loaded. The APU may also supply hydraulic pressure to move flaps or controls, or may supply air to turbine motors that power hydraulic pumps. These different functions can also be accomplished by GPU's, or Ground Power Units, and by "huffers," which blow air into the aircraft pneumatic system.
Aircraft are equippped with documents which spell out which items may be safely inoperative, and the conditions and limitations under which they may be inoeprarative. This document is known as the Minimum Equipment List, or the Configuration Deviation List. This document will invariably include the APU as an item that may be inoperative for a flight or series of flights. It will also clearly spell out any limitations that may apply to the flight, the length of time it may be inoperative, any mechanical work that must be done to the aircraft to make it safe (inspections, deactivation, etc), and so on. Rest assured that if you go flying with an inoperative item such as the APU, it's been very closely examined and deferred in accordance with the MEL. In this condition, it's legal and safe.
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APU
Thanks to everyone for your replies.
I am a very nervous flyer, who has to fly regularly for family/work reasons and I always find your replies very helpful and re-assuring and they help me a lot when I'm airbound.
Maddie
I am a very nervous flyer, who has to fly regularly for family/work reasons and I always find your replies very helpful and re-assuring and they help me a lot when I'm airbound.
Maddie
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An APU basically provides everything the engines provides except thrust. It is a must in most airlines for the APU to be serviceable for ETOPS flights. For the nervous flyers out there, there's still the batteries and the ram air turbine to take over to provide limited services should the APU fail as well.
Ah, 727s come with an APU. It wasn't in the original plan but got crammed in at the last moment.
727s APUs aren't approved for use in flight. On the DC-9 if an engine driven generator is inop you use the APU for the whole flight. We'd leave it on for takeoff from a contaminated runway. On the 747-400 you can use it on takeoff for pneumatics but not electrical power up to 15,000.
727s APUs aren't approved for use in flight. On the DC-9 if an engine driven generator is inop you use the APU for the whole flight. We'd leave it on for takeoff from a contaminated runway. On the 747-400 you can use it on takeoff for pneumatics but not electrical power up to 15,000.
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Operations under MEL (Minimum Equipment List)
It is definitely safe and legal to operate under MEL provisions.
Aircraft operating under ETOPS (Extended Twin engine OPerationS) rules will have a very long and detailed MEL which lists restrictions and limitations that apply when various systems are inoperative. The more flight critical the system is, the greater the restriction placed on the aircraft's operation.
For example if an ETOPS aircraft is flying transatlantic or transpacific without an APU it will not be permitted to fly as far from a diversion airport as it would be with a functioning APU. For the airline this means that they can operate their scheduled flight (and avoid the expense of a cancelled flight or hire in aircraft) but the flights routing will be more expensive to fly than it would ideally be. For the passengers this means that any minimal extra risk involved because the APU is inoperative is accounted for by ensuring that they are closer to a diversion airfield than they would be if the APU was working.
In very simplified lay-man's terms the MEL quantifies the additional risk due to a particular system being inoperative and applies an appropriate restriction on the aircraft's operation. I'm going to stress here that the risks involved have been judged to be insignificant. I'll also point out that National Airworthiness Authorities (FAA, CAA etc) monitor airlines use of the MEL and how quickly airlines rectify MEL defects. If the Airworthiness Authority felt that an airline was taking undue advantage of the MEL they would get into trouble very very quickly and risk having their operating licence withdrawn.
Aircraft operating under ETOPS (Extended Twin engine OPerationS) rules will have a very long and detailed MEL which lists restrictions and limitations that apply when various systems are inoperative. The more flight critical the system is, the greater the restriction placed on the aircraft's operation.
For example if an ETOPS aircraft is flying transatlantic or transpacific without an APU it will not be permitted to fly as far from a diversion airport as it would be with a functioning APU. For the airline this means that they can operate their scheduled flight (and avoid the expense of a cancelled flight or hire in aircraft) but the flights routing will be more expensive to fly than it would ideally be. For the passengers this means that any minimal extra risk involved because the APU is inoperative is accounted for by ensuring that they are closer to a diversion airfield than they would be if the APU was working.
In very simplified lay-man's terms the MEL quantifies the additional risk due to a particular system being inoperative and applies an appropriate restriction on the aircraft's operation. I'm going to stress here that the risks involved have been judged to be insignificant. I'll also point out that National Airworthiness Authorities (FAA, CAA etc) monitor airlines use of the MEL and how quickly airlines rectify MEL defects. If the Airworthiness Authority felt that an airline was taking undue advantage of the MEL they would get into trouble very very quickly and risk having their operating licence withdrawn.
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R.T.S.
T/Cook 757 probably pulled back on stand as would be wise to have a butcher's down the exhaust if it failed on engine start. With an internal airflow/bleed offtake problem wouldn't be a surprise if a few flames popped out the back. 737 MEL calls up an inspection of the tailpipe prior to deferrment if memory serves me right. Depends on type but 10 days is quite normal limit for allowing a/c to op without an '$' APU - not v nice this time of year tho....
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