CF34 3B thrust reversers

[baobab72]

Hi folk
From my notes:
The General Electric CF34-3B engines mounted on the Challenger 605 are equipped with a cascade vane type thrust reverser system designed to assist the wheel brakes and the spoilers in stopping the aircraft during an rto or on landing and basically consisting of translating cowls pneumatically actuated by a Power Drive Unit or PDU with bleed air from the 14th stage bleed air manifold that when the thrust reversers are actuated they will move the translating cowls rearward through a flexible driveshaft and ballscrew actuators and at the same time they will rotate a set of blocker doors into the bypass airflow redirecting it forward through a set of cascade vanes.
T/R operation is controlled by a L and by a R ARMED switch on the THRUST REVERSER panel that when set to the ARMED position will arm the thrust reverser system and it drive the 14th stage thrust reverser valve open, as indicated by an EICAS message. Reverser deployment is then accomplished by squeezing the thrust reverser triggers and uplifting the thrust reverser levers commanding the PDU to drive the translating cowls open – initially the travel of the levers is limited to about 20° by a solenoid stop that locks the levers in idle preventing power from being increased until the cowls reach the full aft position, triggering an amber REV icon in the N1 gauge while the reversers are in transit while as the reverser become fully deployed the icon will turn green.
As safety feature the thrust reverser system is comprised of an emergency stow system controlled by the respective UNLK switchlights on the THRUST REVERSER panel that when pressed it will direct 14th stage bleed air into the PDU returning the thrust reverser to the fully stowed position.
The emergency stow system is used in conjunction with a lock pin and of a lock cam mounted on the flexible shaft that together with a brake will prevent the T/R’s from travelling by more than ¼ of an inch if un-commanded; with an automatic stow system that in the event of a lock cam and of a lock pin failure resulting into the T/R’s moving from the fully stowed position will energize a stow solenoid allowing 14th stage bleed air in the PDU forcing the reversers to return to the fully stowed position; with an automatic thrust lever retard system that in the event of an un-commanded thrust reverser deployment will retard the affected thrust lever to idle. Besides in order for the thrust reversers to deploy a wow signal must be received from the squat switches.

QUESTIONS?
If the PDU is using bleed air both for deploying and stowing the translating cowls, whta exactly happens when you press the emergency stow switchlight? Am i missing something about the functioning of the PDU and of the autostow solenoid?
I guess i am a bit confused about how the system works, and what the bleed air is used for.

Many thanks

[flyboyike]

If the PDU is using bleed air both for deploying and stowing the translating cowls, whta exactly happens when you press the emergency stow switchlight?


I'm not sure how much more you need than what the manual says, which is:


As safety feature the thrust reverser system is comprised of an emergency stow system controlled by the respective UNLK switchlights on the THRUST REVERSER panel that when pressed it will direct 14th stage bleed air into the PDU returning the thrust reverser to the fully stowed position.


Or are you asking what physically happens in the PDU? I can tell you I don't know and I don't care, and I have just under 3,000 hours in CRJ-100/200 with the same engines and same reversers.

I will tell you this: when you're in the sim practicing an inadvertent deployment, make sure your hand is not in the way of the thrust lever that will be snapping back, because is snaps back violently quickly, they have hurt people.

All this is handled much better on the bigger CRJs.

[baobab72]

Hi
Many thanks for your answer, my question verted on how the PDU works, not in details, i was just wondering if it is using bleed air from the 14th stage to both deploy and stow the thrust reversers or if it uses the bleed air to stow them and dumps the pressure to deploy them?
I dont need to know the exact function of the drive unit, just a general understing of how it works!

[flyboyike]

If it dumped the pressure to deploy them, would they all not be deployed when the engines were shut down?

[Yancey Slide]

Maybe the PDU is air pressure from the engine pushing a screw drive connected to the reverser mechanism and actuates each direction depending on which side of the system the air pressure is pushing? Thus it won't move with no air pressure from a shutdown engine? Probably some solenoids to detect position and interlocks to stop air pressure from being allowed into the system depending on system commands....