If At First You Don't Succeed...Try Another Part.

Last week at work we were presented with an airplane that had a problem which should have been a simple fix but in fact turned out to be a real headache. This is the type of thing that happens every now and then that keeps the job interesting.

The plane can in a day before I started my work week. The original call was for a hydraulic leak at the engine. The fitting that was leaking was identified as one from the Engine Driven Hyd Pump. Ti cut off the hydraulic supply to the pump the fire handle for that engine was pulled which closed the gate Valve for that engine. Unfortunately it also set into motion a series of events that ultimately doomed the troubleshooting and nerves of several mechanics and several days worth of lost revenue for the aircraft.

The fitting had a bad O ring on it, which was changed and leak checked. The plane was dispatched but had to return to the airport after the A system "Low Press" light came on about 20 mins into the flight.

That night the engine pump was changed, the filters were changed, the electric hyd pump was changed after it was found leaking as well. As a hunch one of the mechanics checked that Gate Valve which isolates the engine from the rest of the hydraulic system. Turns out the Gate Valve was sticking and not operating smoothly. The valve was R&R'd.

After a leak check the system operated per the MM and the plane was put back into service. The following morning the plane took off and once again came back the the airport with the "Low Press" light illuminated. The engine pump was so hot it had changed color!

Those of you who are mechanics know that a double air return is now a serious issue and that the FAA is going to be checking it out. It also tends to get the attention of MX Control.

The engine pump was changed again, filters, the system drained and that's where I came in. I was working midnight shift overtime and I was working with another guy who I'll call Gold Brick. Gold Brick and I changed the A system Hyd Module, refilled the system and had to stop and wait for parts due in in the morning. I should also mention that the supply and return line to the pump were also changed.

Once the parts came in we installed them and did our leak check. We also ran the engine at idle for 10 minutes to see if anything happened. After this run we verified that there was fluid in the case drain filter. The case drain system is responsible for cooling the pump while it runs. All was good. By now it was Day shift and I was working with another guy I'll call Hearing Aid. Hearing Aid and I figured we would take the plane out and do a high power run for 20 mins to make sure all was ok.

We taxied out to the run up area and powered up to about 73% power and set the clock to time the run. Occasionally we would power up to about 85%. Everything was good until the 9 minute mark. Sure enough the "Low Press" light came on. Hearing Aid quickly throttled back and turned off the engine pump but the damage was done.

We taxied back to the overnight parking area and opened her up. Sure enough the pump was hot, when I opened the case drain system the fluid was hot. By now it was Swing shift and I had to leave.

As it turns out this is not the first time this has happened on a 700 series plane so we were getting help from all over with further trouble shooting. A Tech Foreman flew in and helped the guys on midnight  shift. Again they changed the pump, all the filters, all the lines, all the fluid and finally came to a little check valve which is in the case drain system right before it goes into the hydraulic cooler. That check valve was stuck. Since the valve was stuck the cooling was effectively shut off to that pump which allowed it to run for about 15 mins before it cooked itself to death.

So a little $200 check valve cost this plane a weeks worth of revenue, the cost of six or seven mechanics (several on OT), three pumps, several filters, two sets of hydraulic lines, I would say 15 to 20 gallons of hydraulic fluid and a lot of blood sweat and tears.

The plane flew fine on it's next trip. I know because after all that we decided that we should do a test flight for the plane. I was lucky enough to go on the test flight and monitored the A system pressures. As I said all was good.

It seems that when these things happen it is almost always a little part that fixes the issue. It's one of those things that you remember for your whole career. "Hey, remember that plane with that hydraulic pump....?"