B777 Thrust Asym Comp/ ECS questions

[tom775257]

Hi,
I have a few specific and a few general questions, any answers greatly appreciated...
On the 777, overhead in the cockpit there is a 'Thrust Asym Comp' button. I assume this is for asymetrical thrust compensation...is this correct. Does this equate to, for example, loss of one engine after takeoff with thrust asym comp enabled, and the plane without autopilot enabled the plane will still fly in the same direction due to automatic correction of this without any anti-yaw control inputs by the pilot necessary; or does it simply keep the thrust of the two engines constant through the FADEC when the throttles are slightly unequally pushed forward??? (Does the 777 even have FADECS??)
Second question relates to the environmental control system of aircraft:
Do aircraft contain any active aircon cooling systems. From my limited knoweledge I beleive inflight the engine bleed air is cooled partly through heat exchangers in the engine nacelle and the wing for anti ice, but most of the heat is taken away by a heat exchanger in the aircon packs that exchanges the heat with cold air from outside. How does this work on the ground, especially in hot climates. Do modern aircraft contain a phase change cooling system for use on ground to keep the interior of the aircraft cool(e.g. like a fridge or nomal Aircon system) which I suspect would be far to heavy or perhaps a peltier solid state silicon cooling system (prob too expensive..??)
Thanks for any help, please excuse any mis-use of terminology or ignorance regarding the subjects I am discussing, sadly I'm just a Bio sciences uni student...
Cheers,
tom.

[exeng]

Thrust Assymetry Compensation does indeed apply rudder to counteract yaw following an engine failure. From what I remember TAC applies about 95% of the rudder requirement, it's up to you to put in the rest.

As for the air conditioning system, it is typical Boeing. Air is bled from the engine compressors, pre-cooled and supplied to the air conditioning packs. These packs contain heat exchangers(utilising ram air in flight and air drawn through them by a fan on the ground) and an aircycle machine operating on a boot-stap cycle. In addition a trim air system is used to supply additional pre-cooled only air to the aircraft zones that require more heat. Also recirc fans are utilised. The entire operation is normally automated with master temperature control range set from the flight deck and individual zone temperature control (within master temp range limits) available from the flight attendants station.

Hope this is of help


Regards
Exeng

[tom775257]

thanks alot for the info:

[tom775257]

Just to confirm then:
A modern aircraft taxiing by itself at DFW in the the summer, has no way of activly cooling air i.e. through refridgeration?
cheers.

[exeng]

Tom,

You stated, <A modern aircraft taxiing by itself at DFW in the the summer, has no way of activly cooling air i.e. through refridgeration?>

Yes it does. As I stated in my previous post
'These packs contain heat exchangers(utilising ram air in flight and air drawn through them by a fan on the ground) and an aircycle machine operating on a boot-stap cycle.'

Air conditioning of the aircraft on the ground is essential, particularly in high ambient temperature conditions, hence the fan drawing air over the heat exchangers and the air cycle machine.

Regards
Exeng

[tom775257]

Hi Exeng!!

Just to continue my question, with regards to your answer:
'Air conditioning of the aircraft on the ground is essential, particularly in high ambient temperature conditions, ***hence the fan drawing air over the heat exchangers and the air cycle machine.'***
From my limited knowledge a heat exchager just literally is a interface of two fluids gasses or solids in the same or differing phase with an interconnecting conductor. Hence if you have outside T.A.T. of 100 deg. F. the best with a REALLY efficient heat exchanger, passing hot gasses over this heat exchanger, the minimum temperature you can get the hot gas too is 100 deg. F. due to thermodynamics. ...however my point is this...to get below this imaginary outside temp of 100 deg F. you will need active cooling no matter what simply due to the laws of thermodynamics. My question is how is this achieved..the only types of sustainable self-contained active cooling I know of are
i) Phase change cooling i.e. the use of a refridgerant i.e. R134a, CFC's, HFC's, HCFC's etc. within a pumped system with an evaporator, condensor, strainer and pump; or sometimes used in the world of I.T., solid state silicon 'peltier' coolers...
How is cooling achieved within an aircraft on the ground in hot conditions while not at the gate?? sorry if I am completely off on this point, just using prior knowledge to try and understand.
cheers for any explainations.
Many thanks,
tom.

[This message has been edited by tom775257 (edited 10 January 2001).]

[QAVION]

Hi, Tom.
Although the engine bleed air is cooled somewhat before it reaches the aircon "pack", a lot more processing is carried out. On a 747-400, the air is pre-cooled with the pack heat exchanger, then compressed, then run through the heat exhanger again, then fed through a turbine and then into the cabin (in the latter stages, expanding it greatly). This cooling, compression, cooling and expansion allows the very hot bleed air to provide very cold air to the cabin. Even on 100 degree F days, you may see/feel minute chunks of ice being spat out the airconditioning ducts. The process is not unlike your home refrigerator (which compresses, cools and expands a refrigerant gas/liquid). Other than atmospheric air, however, no gas is necessary in an airplane pack.

On the ground, the cooling airflow over the heat exhanger may be assisted with a fan (In the air, of course, the airflow over the heat exchanger will be more than adequate without the fan...especially at Mach .83 at 39,000'!).

The aircon packs may be shut down during certain phases of your trip (e.g. engine start and takeoff), but hopefully, the cool air in the cabin will not disappear straight away.

Rgds.
Q.

[tom775257]

Hi there,
thanks for the final piece of explaination, I completely neglected of thinking of the fact you are working with a high pressure air feed, so once you get rid of the heat of compression, through allowing expansion you get cooling..
Thanks alot for all the details, that explains it.
cheers,
tom.



[This message has been edited by tom775257 (edited 10 January 2001).]