Hydraulic System

[Rob]

I’m on the lookout for a clean 500A or 500B. Having owned various aircraft over the years, including a 560F, I know these aircraft do require a lot of attention to maintenance. One area that I want to learn more about is the design and care of the hydraulic system. My understanding is that there is an engine driven pump on the left engine, but that’s about all I know. Does anyone have a diagram of the entire system that could be posted or sent to me? Any other comments about the task of rebuilding different components or areas of common problems would be very much appreciated. Perhaps there is a nice aircraft looking for a new home that someone knows about? Thanks in advance!

[appleseed]

2 Eng driven and one elec aux.
I believe this schematic represents all twin commanders after 1960

[donv]

It still amazes me that the Aerostar, even the last ones from 1984, only had one engine driven hydraulic pump with an electric backup.

[Rob]

2 Eng driven and one elec aux.
I believe this schematic represents all twin commanders after 1960

MEGA thanks mr. appleseed! Not sure where I got the idea that there was only one engine driven pump. Now I just need to find an airplane!

[DarylW]

my 1953 520 only has an engine driven pump on the left engine and a hand pump next to the copilot seat
maybe that’s where you got the idea!

[Roy]

Heads up, I was taught long ago by Morris (R.I.P.) after engines start to pull the circuit breaker on the electric pump, and turn it on at short final or outer marker (short final better). If you have a leak inflight and the system depressurizes (recognized by your nose wheel coming down inflight) the engine-driven pumps will suck the reservoir dry. If the electric driven pump is running, it too will suck the reservoir dry. If you disable the electric pump, there will be enough fluid left in the reservoir to drop the gear, have flaps, brakes and steering, at least enough to get off the runway and onto the ramp.

Speaking with 1st hand experience, happened to me on a ferry flight way back when, had to divert to KSGF and perform repairs in a puddle of 5606. Had I not pulled the breaker as advised to do, best-case I would have needed a tow off the runway. Worst case, in the grass possibly hurting things.

Roy

[JimC]

It depends on where the leak is. The electric pump and engine pumps do not draw from the same spot in the accumulator/reservoir. The electric pump can access fluid after the engine pumps are sucking air.
I had a fitting come loose on the RH hydraulic pump and it spit out all the fluid it could pump. There was still enough fluid left in the system for the electric pump to give me full control of all systems on landing.

In other words (aside from the failure) it worked as designed.

[donv]

It does make a difference in where the leak is, but in the 840/980 anyway (and I think the 690), the electric pump draws from a standpipe in the main reservoir. So far so good... the bad news is that in the Commander, the standby pump can power everything except the landing gear. So unless the leak is in the landing gear, which is a possibility, you have a real risk of the electric pump pumping all the standpipe fluid out through the leak as well.

That is why the recommendation for pulling the electric pump circuit breaker. The emergency procedures for the 980 are specific:

SUSPECTED HYDRAULIC LEAK

To minimize immediate fluid loss should a leak exist in the auxiliary pump lines or components supplied by it:

1. HYD PUMP Circuit Breaker - TRIP
2. Flap Control Lever - NEUTRAL

Prior to landing:

3. HYD PUMP Circuit Breaker - RESET
4. Hydraulic Pressure Gauge - MONITOR

The old time guys would typically pull the HYD PUMP CB after takeoff, and put it in before landing, even in normal operations... or at least be spring loaded to do so. Part of the reason for this, I believe, is because in a piston Commander, if you lose hydraulics, you are losing flaps, brakes, and nose wheel steering-- pretty important. In a Turbo Commander, while you use the same things, you have reverse and asymmetric reverse to provide braking and steering, and so it is much less important.

No brakes in a 500, you are quite possibly going to run off the end of the runway. No brakes in a 980, well, no big deal.

[Bruce Byerly]

Exactly Don.  Why I don’t do it in a 500 is because I forget and then have no brakes on start leading to an embarrassing lurch.

Regarding MK, while he was right on this, he really did a big disservice to so many owners over so many years.  I always enjoyed talking to him but beyond being convicted in crimes related to aviation and working on planes without a license, I believe he caused some fatalities and, looking back, I frankly regret not speaking up more clearly to some of his victims. But mom always said, “if you’re not going to say anything nice, don’t say anything at all.” So I never did.

[donv]

When I received my initial training in the 690 (A? B? I don't remember, although I do remember the airplane), the late John Baldwin told me two things about that HYD PUMP circuit breaker.

The first one was about the "old time guys" flying around with the circuit breaker out. This is funny because John was no spring chicken himself--  he flew DC-3s in Korea and would probably be about 100 now (his widow lives down the street from me, and she is a hale and hearty 98!).

Second was his lamentation that in the 840/980, they moved the circuit breaker from the lower left corner of the CB panel, where it was very easy to find, to somewhere in the middle of the panel, where you have to search for it.