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Disclaimer: These IAC pages are developed by individual IAC members and do not represent official IAC policy or opinion.

Collected by: From: (Rob Jacobsen)


As per Guenthers request, here is a editorialized summary of the correspondence and testing which I went through in solving the INVERTED OIL SYSTEM PROBLEM for which I requested your help in May of this year.

The problem turned out to be the O-ring plug in front of the sump screen. Three factors conspired to make this determination difficult and long winded: a). The sump screen was 3/16 of an inch short. b). The O-ring on the plug was to thin (minor diameter). c). The sealing surface upon which the O-ring seated was scuffed.

After the obvious was done the inverted oil system now behaves as it originally did: a). Roll to inverted and the oil pressure recovers in 2 seconds. b). Oil pressure is maintained on the vertical line.

As can be expected I received many good suggestions, some of which were not relevant to my problem, but as they are certain to be relevant to someone else in the future they are included.


This is in the hope that someone out there has had a similar problem and that he/she will be kind enough to share your solution.

I have an Pitts S1E with the standard Christen Inverted Oil System. Recently I was forced to major the 180 HP Lycoming A1A which powers it (as a result of a large crack in the crankshaft at the prop web and less than ideal compression in a couple of cylinders).

At the same time the inverted oil system was cleaned and overhauled, being reinstalled with new hoses everywhere. Everything is back together again and flying, but the oil system has problems. The system behaves in the following manner:

a) In normal upright flight, steady oil pressure of 84 PSI.

b) Roll inverted, the oil pressure falls off to 20 PSI (Red Line) and recovers over 3 to 4 seconds to 76 PSI.

c) Roll upright, the oil pressure gauge flickers and the pressure is back at 84 PSI in a heartbeat.

d) Take it vertical (pull from upright at 140 MPH), the oil pressure initially holds at 84 PSI for about half way up the vertical line, then starts to become erratic ( gauge swings wildly), then it falls to redline (20 PSI, at which point I fly it over the top before it gets any worse).

e) Take it vertical from inverted, same behaviour (except initial pressure is 76 PSI).

Needless to say the problem has been aggressively pursued, the following attempted fixes performed:

a) The O-Ring in the sump plug replaced.

b) The switch over ball valve cleaned, its action tested and the balls seated (was found to be in excellent condition).

c) The oil separator can cleaned and the action of its ball valve checked (was found to be in excellent condition).

d) All hoses carefully inspected for obstructions ( all were found to be in excellent condition with no evidence of any collapse when in their operating positions).

e) The T has been checked for obstructions ( it was unobstructed).

Unfortunately nothing has worked/solved the problem, I know that this is not the first time this problem has occurred to one of us unfortunate souls - PLEASE HELP - What do I do next?

From: Guenther Eichhorn <gei at>

You didn't say whether you have a constant speed prop or a fixed pith prop. If it is a constant speed prop, then maybe my experience will help.

I had oil pressure problems for a long time. In my case it turned out to be caused by two problems: 1. The prop governor was worn out. The oil passed through the governor in large amounts. The oil pressure was kind of low because of that. As soon as the governor had to work, the pressure dropped dramatically. 2. The prop blades were too tight. They seem to get stuck and then moved again. This caused surging of the prop, accompanied by some oil pressure fluctuations.

I hope this may give you some ideas.

From: Warren Anderson <>

You are describing behavior that I see in my Extra 200 and also observed in a Super Decathlon. I thought it was normal.

The Lycoming engine manual prohibits vertical, knife-edge or tumbling flight longer than 10 seconds because of the risk of oil starvation while the inverted valve is floating--neither positive nor negative G.

If the components are in good condition and hoses are unobstructed, double check the mounting height of the inverted valve and separator; one is a critical dimension. Also, obviously check that the engine is plumbed correctly--I believe that a plug can be dislodged or not installed on one of the internal lines. If you haven't already, double check the Lycoming manual inverted oil system installation checklist to make sure you are not leaving something out.

From: Jeffrey Lo <>

I seem to remember that the "T" was the original, less than optimal solution for getting inverted oil (from the breather port). On my old Pitts S1A it got better after I added a B&C alternator and used an oil line adapter there (the vacuum pump pad) where there was more oil pressure available.

From: Tom R. Myers <>

Call Bill Bainbridge at B&C and order the vacuume pad inverted oil system pickoff unit. the problem is the limited diameter of the "T" when switching between halves of the upright/inverted system. Not enough oil can get through in the transition. The pressure dip is a known phenomenon that the B&C unit was designed to eliminate. The "T" is completely eliminated from the system, and the upright and inverted systems have independent oil sources from the case. The B&C ad appears in sport aerobatics regularly.

From: Herman Dierks <>

Are you saying it did not do this before you did the major OH? I am not sure why this is happening now if I was not happening before. Are all the hoses routed the same a before?

As Tom M. noted, it is common to have the dip in pressure when you roll inverted. I had this problem on one Pitts. I simply made my own pickup that goes where the vacuum pump pad goes. This is just a 3/4 inch block of AL the same shape as the vacuum pump pad and you drill and tap it for an An fitting. You do away with the T and connect the line that goes to the ball valve to this fitting. Connect the crankcase vent line directly to the Christen tank.

With the T fitting, the hose is purged of oil by the crankcase vent and each time you go inverted you have to wait longer for the oil to refill this hose. With the hose going directly to the back of the vacuum pump area, this hose remains full of oil when you go back upright.

I have also used the B&C adapter on another Pitts (S1T) as I put the B&C Light weight 8 amp. alternator on the vacuum pump pad and this then requires the special B&C oil pickup to set between the case and the alternator. This plane still has some dip as I go inverted.

The other trick I did on my old Pitts was to put a prop accumulator on the firewall and it is plumbed into the main oil port on the back of the acc. case. This accumulator is charged with about 40 PSI of air (preferably nitrogen) and when the engine starts up it forces about a quart of oil into the accumulator to balance the 40 PSI of air. When you go inverted an the pump has no oil for a few seconds, the accumulator pushes its quart of oil into the system. A bit hard to describe, but this Pitts hardly wiggles the oil pressure gauge when it goes inverted. I buy the prop accumulator from Wag Aero. They are new surplus McCaully parts for $75.00. I bought one for the S1T but have not found a good place on the FW to install it. You can also find these devices in car racing catalogs, like Summit or Racer Wholesale. You can even get them with an electric valve so when you shut off the engine you can close the valve first and this keeps oil in the accumulator. Before you start the engine, turn on the valve and it will push oil into the case (by the oil pump) and pre-prime the pump so you don't have that nasty oil pressure lag when you start the engine.

I have wanted to write this up for SA, but the road to hell is paved with good intentions..

From: "Bert H. Berrong" <>

I have had similar problems on occasion with both a Pitts S2B and a Rebel 300. The fact that your system works fine after 3-4 seconds tells you that all of the mechanical stuff is OK. Suggest you consider the possibility that the problem is oil viscosity which lowers (gets thinner) with higher temps. Get the engine good & hot, try the experiment several times until all of the oil in both the hoses and the residual in the can are good and hot and see if the results are the same. Same experiment goes for oil use when real hot &/or cold. Big time use when not hot, moderate when hot. I use 1-2 quarts for an unlimited sequence in Colorado but only 1/2 qt. in Texas when it is nice & hot.

From: "Dave Pilkington" <>

New hoses everywhere? New line to the oil pressure gauge? Check that the restrictor is still there. The standard oil pressure indicating system damps out the indications of actual pressure changes and fluctuations as well as introducing lag into the system.

I believe that what you are seeing is the real oil pressure in the engine. With the restrictor in the line the indicator lets you think that all is wonderful!

From: "Dave Swartz" <>

How did your oil system behave before the work was done? My stock S2B has always behaved pretty much as you describe (the delay for pressure going inverted is somewhat less). Since the ball valve is mounted (on the S2B) flush with the firewall, it will not be in either the inverted or upright position after you get close to zero angle of attack in a vertical pull. Watch your G-meter when you pull to vertical. It will show positive G (causing the ball to stay in the upright position for good pressure) for a substantial portion of the up-line. Near the top of the line (assuming its a good vertical) the G-meter will show near 0 G and the oil pressure will not be maintained. I have seen installations of the valve where something like a 15 degree offset is used to allow oil pressure to be developed while vertical. If my aircraft was experimental, I would make this small modification. I've never liked the idea that I can't get good oil pressure during the last part of an up line and through most of many vertical down lines but the loss of pressure never approaches the 10 second limitation Lycoming suggests for knife-edge flight (also no oil pressure).

From Dick Rihn

Replacing the "O"ring in the sump plug is not always enough. The plug can partially dislodge. It must be tightly held in place by the finger screen. If the screen gets bent or worn it may not be long enough. It can be replaced with a newer, longer one or you can back up the old one with a "hogged"out washer. To test for this remove the hose from the sump plug crankcase fitting and insert the 10-32 threaded rod into the plug. (Yes, I know it isn't easy) If you can move the plug back and forth that is your problem.

From: Joseph

I had a worse problem when installing a new engine ( AEIO360A1E) and Christen inverted system. READING the product manual from the Christen factory helped me a lot; in this manual they state that depending on the engine oil sump installation, normal or extended, tube mounting etc., several side effects during acro might be acceptable (or normal). So maybe it is not quite as bad as you think.

Did you read this product manual (also the part on trouble shooting), I think this product manual ( for your type of inverted system) can be ordered from the Christen factory .

From: Hal Hichcock <>

The only thing I can think of is that the return lines are blocked, or there is a blockage in the case. My only experience with something like this was where a bearing on the crank was blocked. However, the engine was near seizure after 1 hour airborne.

From: Herman Dierks <>

Did the A&P's that did the OH put back all the same fittings into the engine sump for the inv. oil system? I ask, because I have had a number of these strip out when you try to take them out and then had to replace them. If they just use an AN fitting for example, without the tube that is swaged inside it for the oil pickup (Christen part), then you might not get the same performance. Also, there is a 'plug' that goes at the end of the oil screen at the bottom of the sump. This is needed to force the oil pump to suck oil through the ball valve and not the bottom of the sump. I think if this was left out you would not have any oil pressure when inverted. As someone else noted, the oil pressure fitting that goes at the rear of the engine (just behind the top right dynafocal mount) is suppose to just have a small hole in it (soldered shut and then drilled with a small bit). Most engine shops know to put in the right fitting here. It can be a bitch to get at once on the plane, unless the shop put a 'street El' on it and then the 45-degree fitting.

© Dr. Günther Eichhorn
Email Guenther Eichhorn