Speaking of T-6 hydraulics (a story about a failure)

[Medtner]

I just had a wonderfully complex and nuanced Accusim-experience.

The last days I've been flying the T-6 around Oslo in Norway, and decided to take a trip over the mountains to the west coast, Bergen.
I haven't noticed any problems on the T-6 apart from some slightly dirty filters (noticable when on very high power settings, which I usually am not).

Today I wanted to do some pattern work, and during crosswind I noticed that the plane didn't quite reach normal pattern speed. I noticed the gear was still down, even though I raised it after I had passed the point at which I could have safely landed on the runway if something went wrong.

I tried cycling it, but no luck. The hydraulic gauge moved strangely - in fits, but didn't quite make it away from 1000psi. (the fact that the system may have been modelled "upside down" doesn't matter here - I use it as if it really does have the constant full pressure as I'm used to, until A2A maybe changes it).

I decided to make a careful landing and inspect it at the ramp. During base and final I had trouble with the flaps too - the same twitchy indications on the hydraulic gauge, and the flaps only came down 20-25 degrees, and slowly at that.

After a very careful landing (in case the gear locks had come out, even though I could see them locked in the wing-windows) I taxied to the ramp.

Indeed - a slight hydraulic leak, but not much to speak of. Having fixed the leak, I fired up the engine intending to do my planned pattern work. I couldn't raise my flaps. Still something wrong? Hmmm... It took a few minutes before I realised that I had to fill up the hydraulic tank too - it was almost, but not quite, empty.

During the next successful patterns I noticed how fast the gear comes up or down - it has not been that fast the last several hours I've flown. This leak has crept up on me without me noticing before it was critical. I always cycle the flaps during runup to check that the system works, but I guess this time I used whatever fluid was left to do this.

So much fun!

A question however: When almost, but not quite, empty of hydraulic fluid - would the pressure still be able to reach 1000psi, but at the same time not be able to raise the gear or lower the flaps? My instinct tells me that the pressure should drop as there was less fluid in the system to make pressure with. If that is the case I would have had an indication on the gauge before takeoff that something was wrong, but in my case the only difference was that the pressure twitched (when trying to raise the gear or lower the flaps) rather than dropping several hundred psi like is normally does.
Is this normal, given that the system is a "constant 1000psi"-system?

[crippy]

Wow very cool story!

I still miss very much the pre-flight inspection for the war birds. It feels...."Less real" or less my own bird, when I can't walk around and check the tanks, oil, etc like the G.A. birds.


My story: Not sure if it was an FSX fluke or what happened.
I had the P51 Civilian up and about one day doing some gentile aerobatics. When I got into the traffic pattern, I suddenly lost my aileron control and could not get them to move like the control lock was on. I managed to land with only rudder and elevator control. Once I got on the ground the ailerons started working perfect. No indication of issues in the Maintenance hanger.

[AKar]

A question however: When almost, but not quite, empty of hydraulic fluid - would the pressure still be able to reach 1000psi, but at the same time not be able to raise the gear or lower the flaps? My instinct tells me that the pressure should drop as there was less fluid in the system to make pressure with. If that is the case I would have had an indication on the gauge before takeoff that something was wrong [...]

If you think about it, you realize that it really is not the case. Hydraulic pressure is not a function of hydraulic fluid quantity in the reservoir as long as this is "sufficient".

We take a "constant 1000 psi" system for example, to be generic enough. The pump in such is of a constant displacement type in small applications. This means it always produces the same amount of flow per revolution. This fluid always needs to have a place to go, otherwise you'd burst a line or break the pump. The pressure regulation is achieved by spring-loaded relief valve. It opens at roughly 1000 psi, allowing the flow to return to the reservoir. If you increase the RPM a little for example, the flow increases, the pressure attempts to increase, and the relief valve opens some more to allow the increased flow to return while retaining roughly 1000 psi in the system upstream.

Importantly, as long as the pump gets reliable feed of fluid, it keeps on pumping. When it starts not to, essentially bubbles are passing through it. Typically this shows in sudden drops in pressure, not in an even decrease in pressure (the system having pressure accumulators in it behaves a bit differently).

[...] but in my case the only difference was that the pressure twitched (when trying to raise the gear or lower the flaps) rather than dropping several hundred psi like is normally does.
Is this normal, given that the system is a "constant 1000psi"-system?

It is common that the hydraulic actuators change in their displacement as their position change. This is why one sometimes needs to check the hydraulic fluid quantity only when flaps are up for example. If the fluid quantity is just on the edge, moving an actuator can cause the fluid in the reservoir to be depleted, and pump's failure to, well, pump. How this would show up would kind of depend on a few factors, but generally the pressure either would collapse, and maybe twitch around some very low value, or simply just remain fixed somewhere if the fluid is trapped against no-return valves.

When operating normally, the normal, observable drop in pressure is caused by reduced resistance to the flow when the actuator starts moving. You can think an analogy: imagine yourself pushing against a bookshelf you try to move. When it suddenly starts moving, you can almost tip over, as it stops pushing against you. In our hydraulic system, the relief valve would close due to the drop in pressure. The flow now goes into the actuator, moving it. (The transients at this stage are quite complicated.) When the actuator is moving, the only thing making up the pressure to the system is the force of the part that the actuator moves against the actuator. So: the speed at which the actuator moves is determined by the pump's RPM, whereas the pressure in the system is determined by the force needed to move it. Kind of backwards to the intuition.

(And again, systems with accumulators in them behave differently, and further, "big plane systems" typically use variable displacement pumps. These are entirely different animals. They employ a proportional control of the flow against the output pressure, so that for instance when the pressure is 3000 psi, zero flow is produced, and as the pressure drops, the swash-plate angle is increased in proportion, so that full nominal flow is reached at 2850 psi for instance. These systems always use accumulators as well.)

-Esa

[Paughco]

OK, we don't have the walk-around preflight for the T-6. Whenever filling the gas tanks via Shift+4, I also click on the oil and the hydraulic points on the airplane diagram. I figure that if I had a real T-6, I'd check and replenish oil and hydraulic levels before each flight. Also, I'd replace the hydraulic pump at the first sign of trouble. I do this IRL on my 1950 Harley FL and my 2016 BMW S1000XR.

Seeya
ATB

[AKar]

OK, we don't have the walk-around preflight for the T-6. Whenever filling the gas tanks via Shift+4, I also click on the oil and the hydraulic points on the airplane diagram. I figure that if I had a real T-6, I'd check and replenish oil and hydraulic levels before each flight. Also, I'd replace the hydraulic pump at the first sign of trouble. I do this IRL on my 1950 Harley FL and my 2016 BMW S1000XR.

And it is an essential practice, even if not always explicitly required, to record the amount of fluid added. This is important with airplanes that are flown by many people not necessary much in daily contact. Adding a bit of hydraulic fluid one day is fine in many airplanes. However, if you note that the system was just topped a few flight hours ago, you'd say 'okay, let us check this out'.

For most GA's, this best applies to the engine oil. These engines do burn some, but a sudden increase in oil consumption should be a call for investigation.

-Esa