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Air brake ON at coldstart ... why?


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Hi guys,

 

am I the only one who hates having the airbrake ON by default at cold start? How many time I forgot those before taking-off ... god knows.

 

Why it is this way?

 

Is it because the F5 is usually left this way to avoid stress on the component? Just wondering why!? That thing just not make sense to me so I wonder what I am missing.

 

The excuse "because the airbrake was on from the previous landing" does not hold since dozen of other switches where ON in the last landing as well ...to find them back in the correct initial position at the next cold start.'

 

Please enlighten me ...

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For the shutdown procedure you open them so the ground crew can access the inner workings. Same goes for the F-86. I imagine it relieves hydraulic pressure but don't quote me on that.

 

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Also you probably are the only one that "hates it" lol. Get used to it and add it to the checklist.

 

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Also as the hydraulic pressure bleeds out of the system naturally over time, the air brake would still droop down come the next morning.

 

I kinda doubt this. In the case of the speed brakes hydraulic pressure is not used to hold the control surface in place. The control valve moves on command from the speed brake switch, directing hydraulic pressure to open or close the brake as required. When the switch is neutralized, so is the valve. The hydraulic fluid downstream of the valve holds the brake in place, since one of the greatest advantages of hydraulically operated systems is that the fluid (MIL-H-5606/83282 in this case) is incompressible. Hydraulic components drooping overnight have nothing to do with hydraulic pressure bleeding down, but rather with wear over time in the valves allowing hydraulic fluid to leak by. If the speed brakes opened simply because the hydraulic pressure bleeds down, they would start to open shortly after the engines are shut down, since hydraulic pressure drops almost immediately in that case. (I will admit that the -1's diagrams on the hydraulic system are a little weak, so if anyone has a maintenance manual that contradicts any of my statements I would really like to see it. That isn't sarcasm, I actually really enjoy systems study).

 

We arrive at the cold airplane in DCS with the brakes open, just like in RL, because TO 1F-5E-1 directs the pilot to extend the speed brakes in the after landing checklist.

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  • 2 weeks later...

Last time I touched a F-5 (ex-crew chief) was 1982. If memory serves me right. The reason the speed brakes are open after landing is:

1) Inspect the actuators and hydraulic lines for leaks

2) Inspect the wing root area for cracks

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  • 9 months later...

With the speed brake switch in the open or closed position the valve allows system pressure to reach the open or close side (respectively) of the actuator. If system pressure is lost in flight with the speed brake open it will blow toward closed, but may not completely close. When pressure goes to zero after engine shutdown I don't see why they would not eventually droop. With the switch in the center (off) position the valve is closed and isolates both sides of the actuator, and the speed brake should in theory hold whatever pressure is on the actuator and remain at the angle set. In reality after some time pressure could tend to bleed off, and if you had cracked the brake open to an intermediate position it could slowly creep toward closed due to air load, and you would occasionally have to give it another press aft to get the brake where you wanted it.

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In reality after some time pressure could tend to bleed off,

 

There is no significant pressure to bleed off. Hydraulic fluid is incompressible. You cannot "trap" hydraulic pressure in a closed system of lines attached to an actuator. Pressure doesn't hold the brakes up, the concept of trapping an incompressible fluid on two sides of an actuator does. I don't disagree that the brakes would droop over a long period of time due to mechanical imperfections of the valve but the argument from post #6 of this thread I was rebutting is that they would droop to full extension overnight.

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But you are stating that the fluid is trapped, and I don't see how it is. I don't think the brakes are held up by trapped pressure behind a closed valve. In the closed position the valve is actually open, and is routing utility system pressure to the close side of the actuator. When that system pressure goes to zero there is no pressure trapped in the actuator because the valve remains open. I seem to recall that losing the utility system in flight could cause speed brake droop, but I am getting along in years now. But consider that if pressure remains trapped against the close side of the actuator when you move the switch to extend, how would the system pressure move the actuator against that trapped incompressible fluid? I can't see how one side or another of the actuator is closed off, unless the SB switch is in the off (centered) position.

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But you are stating that the fluid is trapped, and I don't see how it is. I don't think the brakes are held up by trapped pressure behind a closed valve. In the closed position the valve is actually open, and is routing utility system pressure to the close side of the actuator. When that system pressure goes to zero there is no pressure trapped in the actuator because the valve remains open. I seem to recall that losing the utility system in flight could cause speed brake droop, but I am getting along in years now. But consider that if pressure remains trapped against the close side of the actuator when you move the switch to extend, how would the system pressure move the actuator against that trapped incompressible fluid? I can't see how one side or another of the actuator is closed off, unless the SB switch is in the off (centered) position.

 

OK, now we are starting to get somewhere. In the case of an unpowered aircraft, I don't think the position of the SB switch is at all relevant. If this is any sort of typical setup, the spool of the 3 position hydraulic valve can only rest in one position (think of a normally closed or normally open solenoid). Electrical impulse is used to select the other two positions, but as soon as electrical power is lost the valve will move to its neutral position. So the question becomes, where does the SB valve "rest"? In the EXTEND / RETRACT / or OFF (centered) position? 1F-5E-1 doesn't speak to this at all, that was why I asked in Post 7 if anyone has a maintenance manual...

 

On another note, welcome to the forum Statro, it is always nice to have someone here with actual F5 time. If I may ask, because the -1 isn't clear on this either, in a single seat F5 did the SB switch on the throttle spring to center or would it hold position in either EXTEND or RETRACT?

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I don't have any F-5 time, I just have lots of T-38 time. But from my reading of the F-5E Dash One, I believe it is exactly the same system. The rear cockpit switch (F-5F also) is spring loaded to OFF (center) and overrides the front cockpit switch. I recall the front cockpit switch would remain in the OFF or IN position as set by the pilot, but I'm not sure if the OUT position was momentary or would remain in OUT. I did most of my flying from the rear cockpit, but I think it would remain in OUT. I recall with the front switch to OUT the brake remained solidly all the way out, while with the switch OFF in either cockpit they could tend to slowly bleed in over time. On a fingertip formation descent the wingman would sometimes need to drag partial speed brake if lead stayed clean and was too close to idle power, and if it was a long descent you would have to crack the switch from OFF to OUT occasionally. As you said, the unpowered position of the control valve is the real question here. I recall it as being spring loaded to CLOSE (valve open), so that with an electrical failure the speed brake would automatically retract to a fail safe position. You don't want it stuck out when you might have to do a go-around or are down to one engine.

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As you said, the unpowered position of the control valve is the real question here. I recall it as being spring loaded to CLOSE (valve open), so that with an electrical failure the speed brake would automatically retract to a fail safe position. You don't want it stuck out when you might have to do a go-around or are down to one engine.

 

I've always seen the failsafe function of hydraulic speed brakes accomplished with a separate normally open valve that deenergizes open on electrical failure, bypassing the SB valve right to the return, but obviously none of those aircraft were of the F5/T38 family. If you recall the unpowered position being CLOSE, absent a maintenance manual I certainly have no room to disagree. Thanks for the input.

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If it was unpowered CLOSED, not neutral, than the Brakes would eventually drop open as the weight of the doors slowly moved fluid through the valve back into the reservoir.

 

And would also be why you start up with the switch in Neutral so that as hydraulic pressure comes online the doors don't just randomly close themselves potentially catching someone or something. And also why you don't put the speed brakes in the Closed position until you are clear and ready to taxi.

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