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Posts: 97
Joined: Sat Aug 20, 2005 7:32 pm

Fokker 50 Engine Air Duct

Sun Mar 25, 2007 7:18 am

While on a SAS flight a few days ago, I observed an air duct under the engine which changed
its configuration throughout the flight. The device was fully open during take-off, and partly open
during rest of the flight. Can anyone describe the function of this PW125B duct, how it is controlled and
why it is needed on the Fokker 50.
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Joined: Sun Oct 01, 2006 12:19 am

RE: Fokker 50 Engine Air Duct

Sun Mar 25, 2007 5:19 pm

What you observed was probably the oil cooler air exhaust door. There are actually two doors in the bottom engine cowl. The lowest most forward (larger) door is the above-mentioned oil cooler air exhaust door. The second one above and behind it is the engine air intake bypass door. The engine air intake bypass door is adjusted, depending on the required airflow to the engine intake at different power settings, speeds, temperatures and altitudes.

Assuming you observed the oil cooler door, here is how it works:
The oil cooler air exhaust door is electrically operated and controls the airflow through the air cooled oil cooler. The door has two positions, open and closed. With the door in the closed position, there is still a gap between the door and the bottom cowl (which you described as being partly open) to provide a flow through the oil cooler.

The door is normally "closed", when closed it is flush with the bottom cowl. The door opens when take-off power is set on the ground and stays open a certain amount of preset time after lift off through a slow release time delay relay. When in flight and functioning of the time delay relay is completed, the door is closed because then there is sufficient airflow available to the oil cooler.

On the ground below take-off power, an airflow through the oil cooler is created by an ejector. The ejector is driven by "P 2.5" compressor air, controlled by the handling bleed valve. This valve is air operated and electrically controlled by a torque motor. In flight and on the ground with take-off power selected, the engine electronic control (EEC) controls the position of the handling bleed valve. On the ground below take-off power, the handling bleed valve goes to the open position through a handling bleed valve override relay.

I hope this answers your questions.

Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Fokker 50 Engine Air Duct

Wed Mar 28, 2007 12:09 am

I agree on the oil cooler door operation, but isn't the bypass operation the same as a Dash8 with PW120 series engines? In the Dash, the bypass door is either open or closed, depending on a switch selection in the flight deck. It is used in icing conditions to allow ice shed by the intake boot to go overboard, rather than down the intake.
Posts: 91
Joined: Mon Jun 13, 2005 11:08 pm

RE: Fokker 50 Engine Air Duct

Wed Mar 28, 2007 7:04 am

The oilcooler door on the F50 is controlled electronically through the powerlevers and the auto feather switch in the cockpit for manual control. The oilcooler gets its air through a small airintake below the engine airintake.
Also, there is only 1 movable door on the F50.....the engine airintake bypass is a fixed opening and not adjustable.
Appart from that, Stargliders explanation covers it pretty well.

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Joined: Sat Mar 13, 2004 7:52 pm

RE: Fokker 50 Engine Air Duct

Fri Mar 30, 2007 6:10 am

Quoting 2enginesonly (Reply 3):
The oilcooler door on the F50 is controlled electronically through the powerlevers and the auto feather switch in the cockpit for manual control.

As with any Fokker it all depends on customer demands at the time and/or the mod status afterwards.
If I remember correctly standard F50's were fitted with an 2 channel ERSP(Engine Rate Select Panel) as thrust panel.
In this case there is very little logic to control the oil cooler door.

On ground ? = yes
Take off ? = yes
Engine fail ? = No
Door open for 30 seconds then closed again.
The Power levers are used as an input to determine Take of mode or not.


Simplified schematic:

This all changes when there is a 3 channel PMP(Power Management Panel) with a lot more logic installed instead of the ERSP.
Now the Oil Cooler door is operated by channel C of the PMP in normal circumstances, an override may come from channel A and/or B and PLA is just an PMP input.


Simplified PMP schematic:

To cause a little more confusion 
We have customers who decided to change from ERSP to PMP thrust mode control without adapting the oil cooler door.
In this case the PMP is hard wired to leave the oil cooler door operation the "old" way and the PMP will just take over the thrust management.
Forgot the ERSP vendor(probably Honeywell or Smith), the PMP is a Fokker development.

More confusion 2.
The PMP comes in 2 different versions both with 2 different sub versions.  Big grin
With or without auto approach speed control for example.

[Edited 2007-03-29 23:33:02]
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