mark.bitterlich(at)navy.m
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 Posted: Mon Sep 25, 2017 9:53 am Post subject: [Non-DoD Source] Re: YAK 52 - Procedure for emergency gear |
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Yes can be a reason not to move it to the down position Walt. Give me a second. First I'd like to add one more tidbit that is not mentioned in any of the POH's. I managed to figure this one out while ferrying a YAK-50 from East to West coast about 15 years ago.
Dennis mentioned that when both gear levers are in the middle, all air is relieved downstream of the landing gear actuator. "Vented" is a better word to use here. But what this also does is to isolate the whole landing gear pneumatic system from the main air tank, and the compressor and lines. Why is this important to know? Because let's say you have an internal actuator leak downstream of the gear actuator levers? Landing gear selected up, or landing gear selected down, there will be air leaking and it can be so drastic as to drain the main bottle completely. If you isolate the landing gear with both (or one in the case of the YAK-50) landing gear handles in the middle position, you give time for the main bottle to fill up again completely. In the case of the YAK-50, you actually can fill BOTH main AND emergency bottles up this way. Both bottles are plumbed to the compressor on the 50, unlike on the 52 where only the main bottle is filled from engine compressor air.
In any case, you can let the pressure fill back up and then try to make a second attempt to lower the gear normally. You can do this as many times as you want, if you feel this is the right thing to do.
In the case of the emergency system, once you open that valve, you cause shuttle valves to change position on each landing gear. Two on the 50, three on the 52. That shuttle valve has two positions, emergency or main air supply. Anyone who has tested the emergency system knows that you must drain all the emergency air out of the lines, and then select DOWN on the landing gear actuator lever in order to cause the shuttle valves to move back to the position where MAIN AIR will be used to lower the gear.
If you lower the gear using the proper emergency procedure (with both gear handles neutral) air on both sides of each gear actuator is vented, so when air is applied via the emergency valve, there is no air pressure on the opposite side of the actuator at all, that would in any way impede full and rapid movement of the actuator and thus the landing gear. Once the gear is down, it is procedure to keep air applied on the down side of each actuator, which is what is happening at this point, unless there is some gross leak.
If you now move a landing gear handle DOWN, you will supply MAIN AIR (assuming you have any) to the shuttle valve on each actuator. So now you have two air supplies fighting each other to set the position of the shuttle valve, which I submit no one intended.
Let me give you a real world YAK-50 example. Chevron seal in one actuator fails. Gear is selected down. Gear does not come down and lock. Main air drains to near zero. Gear handle is put in middle position and emergency air valve is opened. Main gear locks down, but emergency air bottle drains to zero. In the meantime main air bottle fills back up. Landing is made with main bottle full with brakes being normal. If the brake handle had been selected down, it's hard to prove what might have happened, but if the shuttle valve changed positions, then the main bottle would have started draining.
So it's a complicated question, that requires full understanding of how this system is plumbed and it differs between a YAK-50 and a 52 but understanding what air goes where and why, along with a complete understanding of those shuttle valves is important for all of us to grasp.
Mark
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