Monday, March 07, 2011

Pressure

I have four days to learn all the systems and SOPs for the Screaming Whippet and its larger cousin, I guess that must be the Screaming Greyhound. Then they'll schedule flight training and tests, and on passing we will have our job offers finalized and be paid for the time we have spent in school and training. So, no pressure or anything, right? More like pressure down over the top and around to every corner. And pressure that can cause action by going around corners is what the hydraulic system is about.

The hydraulic system provides motive force for the flaps, nosewheel steering, normal landing gear extension, and emergency landing gear extension. It contains one approximately 2.5 L reservoir, serviced with Brayco-882 hydraulic fluid and pressurized to 31 psi with regulated P3 bleed air. It has a relief valve on top which will release pressure over 45 psi. The reservoir supplies fluid through emergency shutoff valves (which are only to be used in emergencies, because otherwise they cut off the fluid supply to two engine driven hydraulic pumps which will be damaged if they are working away trying to pressurize the system to 2000 psi but there is no fluid for them to pump. The high pressure fluid from each hydraulic pump continues through a line containing a low pressure sensor. The sensor illuminates a low hydraulic pressure annunciator at or below about 1250 psi. Both hydraulic pumps must be working to depart, but it's okay if one of the low pressure switches is broken. If a low pressure light is on after both engines are started, you have to figure out if it represents a bad pump or bad sensor.

You can't do this by looking at the hydraulic pressure gauge, because it is located downstream of checkvalves, after the two lines from the two pumps have been merged together, and either pump alone is enough to provide 2000 psi to the system. And you can't do it by momentarily selecting the hydraulic pump off on the good side, because I was serious when I said the hydraulic pumps were to be shut off only in an emergency. So you have to do it by shutting off the engine on the good side. (You do this only on the ground, because if it happens in the air you don't need to worry about being good to depart: you've already departed). With one engine running, if that side's low hydraulic pressure warning light is still on and the system hydraulic pressure is still at 2000 psi, then obviously there is nothing wrong with the pump and it must be the pressure switch or something in the low hydraulic light circuit that is unserviceable, and you are allowed to depart. You have to remember to restart the other engine, though.

The system also includes an accumulator (a buffer for sudden high demand) with a 750 psi nitrogen precharge. There is also a high pressure relief valve if the pressure manages to exceed about 2300 psi, and return lines from the pressure relief valve and the various services, back through a filter to the main resevoir. The reservoir, accumulator, filter and the flap and landing gear selector valves are all located outboard in the left engine nacelle. The engine-driven pumps are in the engines, with the shut-off valves nearby. The pressure gauge and warning lights are in the cockpit, where I can see them.

Each leg of the landing gear has two dual (i.e. up and down) action hydraulic actuators. Both are required to make the gear come up, but only one is required to put the gear down, thanks to assistance from gravity and airflow for the latter task. (It's forward-retracting gear, a first for me). Thus for normal landing gear extension, the landing gear selector valve sends pressure to only one actuator on each gear leg. The extension side of the other three actuators are connected to another parallel section of the hydraulic system that I haven't mentioned yet. In addition to the lines leading to the engine-driven hydraulic pumps, there is a standpipe that holds about one litre of fluid and which connects to an emergency hand pump, and thence to the extension side of those second landing gear actuators. If the pressure in the normal side of the hydraulic system is too low (below about 250 psi) to extend the gear, the pilot turns on the emergency landing gear selector valve, turns the emergency gear release lever, and ensures the gear is fully extended by pumping a handle in the cockpit. A shuttle valve causes the hydraulic pressure gauge to always display whichever is higher, the emergency or normal system pressure.

There is one dual-action actuator for each flap, and the flaps are interconnected such that either actuator can move both flaps through their full 36 degrees of travel, making asymmetrical flap deployment an impossibility unless the airplane is first disassembled. There is no provision for emergency flap extension or retraction. If the normal hydraulic system fails, the flaps are locked where they are by the flap selector valves and the flap lock valves. If electrical power (to operate the valve) or hydraulic power (to operate the flap actuators) should fail during flap movement, the valve fails to the closed position, locking the flap where it was.

Nosewheel steering I will address in another post.

17 comments:

david said...

Hey, my Cherokee's landing-gear system is complicated, too. I have to make sure there's ... uh ... air in the tires, *and* in the struts. Sometimes I even use a tire-pressure valve (not on the struts, though).

So there.

Paul B said...

"I have four days to learn all the systems and SOPs"

... and I bet you're loving every moment of it, aren't you? Writing about something, as you are doing in your blog, is a great way to learn: if you can explain it to US, then there's a good chance you also understand it!

Are you allowed to tell us mere mortals whether this new aircraft still has propellors on? Or are you hoping to "take the step up" to a jet?

Good luck!

Captain Dave said...

Ah, brings back memories of training in the screaming me-me. The engines are awesome.

Cedarglen said...

The last several posts suggest a wide-open door to a 'real' job, ahem - driving airplanes. I've got a pretty good idea of the details, but my lips are sealed. Pay attention here, good woman! This is WHAT YOU DO! Jumping through a few hoops and learning those systems is part of the job - or in this case - part of getting the job. You are a professional, so you will do just fine.
With mega I understand about the need to WRITE (or talk or teach)to enhance your mastery of the material. I too ma that kind of learner and it used to drive family/friends nuts. There is no need to explain or debate and most of your readers understand that component of the learning process. While the curve is a 45-degree climb, you are already demonstrating mastry of the material. Go For It! Yes, the 'Whippet' family is an interesting breed. You will master it quickly and then wonder why you ever had any reservations about 'Whippets.' Fly, Lady FLY!!!
-Craig

Devil in the Drain said...

Man, I *love* me this blog for lines like "You do this only on the ground, because if it happens in the air you don't need to worry about being good to depart: you've already departed".

Also, Paul B. -- all I know about planes is that it's best to stay inside while in the air. But I bet we get to find out whether it has propellors when she mentions, after landing in some unlikely far-off place, whether she's trying to find the Avgas or the Jet A.

Sarah said...

Heh. Devil in the Drain: Yep. My favorite line was: ... and you are allowed to depart. You have to remember to restart the other engine, though.

The drawback of writing in such detail is the many clues as to which aircraft. I'm pretty sure I've got it, but won't post guesses in comments. Let's not make Aviatrix keep the interesting details to herself.

Devil in the Drain said...

Sarah -- And that was my other favorite line. :-)

But what *would* happen if you didn't restart the other engine? Would you start going in a circle on the ground?

Aviatrix said...

Would you start going in a circle on the ground?

It varies from aircraft to aircraft. I've taxied various aircraft with one shut down and it's typically possible but can be tricky turning towards the operating engine. Best way is to get going straight, pull back the throttle and turn with the nosewheel steering, then bring up the power as you complete the turn and before you run out of momentum.

Some airlines shut one down as SOP to taxi in, and in the C337 the "you have to remember to start the other one" is no joke. Pilots have attempted takeoff on one engine, so the rule is to always start the back engine first. It's hard to forget to start the other one when the forward propeller is stopped in front of you.

pez said...

A friend has been following your blog for years, and sends me links to your posts every now and again. I've enjoyed reading your well written, informative, and often amusing posts the times I've stopped by.... Especially the post a few weeks ago re: the office job vs. a flying job: click here

Now back to the horrors of the corporate world myself (hopefully just for winter), I have to bite my tongue frequently, and hate little more than coming into the same office and sitting at the same desk every day, listening to the same corporate doubletalk. It's soul crushing.

As others have mentioned, I have a fairly high degree of certainty as to the aircraft type and to an ever so slightly lesser extent, the operator, but won't mention them here. If it's who I think it is, sounds like a good gig!

Time to start following this blog regularly, methinks.

All the best at the new job!

-pez

A Squared said...

But I bet we get to find out whether it has propellors when she mentions, after landing in some unlikely far-off place, whether she's trying to find the Avgas or the Jet A.

Whole bunch of planes with propellers fuel up at the Jet-A pump

david said...

What @A-Squared said: the next step up from piston is to turboprops, not to turbofans. Lots of helicopters also run on Jet-A.

flightless bird said...

We taxi in on one engine, shutting the right engine down three minutes after landing.

If the taxi for departure is long enough (such as at Madrid, Amsterdam or possibly Barcelona) then we can also taxi out with just one engine running and try and time the second engine startup and completion of checklists to be ready by the holding point (it take 3-4 minutes for the whole process).

It doesn't always work out as planned though, in which case apparently the best thing to do is blame it on the cabin not being secure for takeoff.

In theory departing without starting the second engine could not happen because of all the checklists and checks that would enable you to pick this up before taking off. To be fair an awful lot would have to be missed for a one-engined take off to even be attempted.

Anonymous said...

With your command of the hydraulics, you need never be out of work again :-)

All the big Citroen cars from the 1950's to the late 90's had a complex system, almost exactly as you describe, they had adjustable ,self-levelling suspension, power-steering which varied with speed, and power-brakes, operated by a rubber mushroom on the floor in the earlier cars....yes, there was also a high-pressure accumulator (green, like a large grapefruit,by any chance?)

and a priority valve, in case of pressure-loss,which sequentially isolated the circuits.
You'd make a good living from the historic car buffs.....oh, the system was licensed by Rolls Royce/Bentley
and had 2 pumps as well!(they were V-8 engined so that's sort of similar to 2 , as well.....if the engine quits and you drain the pressure-accumulators before stopping, it can be VERY "interesting."
Pleased you are progressing towards flyinglate 20 th. century technology- hopefully the "office" will be more comfortable than those you've lived in to date.

verification word "joting" sort of sums it up.

Cedarglen said...

It is YOUR learning curve and your method: Go for it. As before, I understand the method and I've used it. Obviously, you know the detailed material, so you will do jus fine. KNow that many friend are with you! You have support. I won't address gender issues in this case, beyond noting some sympathy for the single male. (After 30+ years as Male RN and professor, I've heard it before... Boys and girls CAN get along, even in the tight confines of a flight deck - or an operating room. Outside those spaces, it can be difficult. You are a professional and I hope that your professional standards always reign. Fingers crossed and a prayer or two, I hope that this connection works for you. -Craig

Devil In The Drain said...

Whole bunch of planes with propellers fuel up at the Jet-A pump

the next step up from piston is to turboprops, not to turbofans. Lots of helicopters also run on Jet-A

@david and A Squared: Oh. Then I guess we still *won't* know, except for the professionals, who *already* know :-)

A Squared said...

Devil in the drain, For what it's worth, the question as to whether it (normally) burns jet fuel of Avgas has already been answered.

Anoynmous said...

We know whether it has propellers now too. The number of blades is still unrevealed, though.