Aircraft Maintenance Tips: Lear Romec Fuel Pumps – A Common Misconception

An important skill of the aircraft maintenance engineer is the ability to troubleshoot, diagnose and correctly rectify issues involving the various systems and components of an aircraft.  Perhaps one of the more challenging of those systems is that of the fuel injection system.

Information on the basic aircraft fuel injection system is available in aircraft maintenance manuals and other engineering textbooks so it needn’t be covered here.  This article aims to focus specifically on the RG9080 series Crane Lear Romec engine driven fuel pump, however the same principles also apply to the RG17980 series too.

A common incorrectly rectified fault is the classic "low pressure at low RPM".  

The scenario would likely occur as follows:

1. A complaint is received from an owner/operator regarding low fuel pressure.

2. The maintenance engineer would carry out standard inspections looking for blocked filters, contamination or other restrictions/obstructions.

3. The next step may involve teeing in a known accurate pressure gauge to directly measure fuel pressure in case of a faulty gauge.

4. If no issues are detected then the engineer would probably adjust the relief valve.

5. Upon carrying out a ground run, the fuel pressure at max RPM would approach or exceed the maximum allowable.

6. More often than not the pressure at low RPM will not have improved sufficiently, if at all,  and if the pump were adjusted until low RPM pressure improved, the top end pressure would likely be well off the scale.

The maintenance engineer may have a good working knowledge of the fuel injection system but it must be understood that the relief valve setting is generally bench set at repair or overhaul and that it is done so with the pump rotating at its specified maximum RPM, not at idle.  

Each pump part number refers to different characteristics of the same basic pump assembly such as direction of rotation, length of drive coupling and relief valve setting.  For example the RG9080J4A & RG9080J7A pumps used on a Piper Chieftain have the same relief valve setting of 38 PSI @ 2575 RPM flowing 42 GPH, but the 4A rotates clockwise (when looking at the back of the engine) and the 7A rotates counter-clockwise.  The RG9080J1 pump used on Shrike Aero Commanders has a short drive coupling and is set to 15 PSI at 2000 RPM flowing 22 GPH.  In every case the relief valve output pressure is set under specific circumstances relating to output pressure, flow and RPM - more specifically the setting is not achieved at idle RPM.  

The condition of the internal parts of the pump directly affect overall pump performance.  Worn or damaged blades will affect the ability of the pump to provide sufficient volume to varying degrees at low or high RPM, however at high RPM the pump is producing far more volume than the engine requires and thus the effects of worn blades are not as evident.

A lesser known (and probably more common) cause of low pressure at low RPM is the relief valve.  At high RPM the relief valve is relatively open and modulating to balance fuel pressure with relief valve spring pressure.  A portion of fuel is recirculated via the relief valve to maintain the desired output pressure.  At low RPM the relief valve is relatively closed and modulating close to or on its seat.  A much smaller portion of fuel is recirculated as the pump now requires more of its total volume to maintain output pressure. The smallest of contaminants between the valve and its seat can severely degrade the ability of the pump to produce any pressure at idle.  

Sources of such contaminants can be fuel tank grit caused by degraded fuel cap seals or refuelling in dry, windy conditions; rubber particulates from degrading engine hoses in ageing aircraft, removing or installing fittings in the system or careless filter inspection/maintenance practices.

Contamination is not the only cause for relief valve induced low pressure at low RPM.  The physical condition of the relief valve and its seat has a large effect on pump performance, particularly if the pump is old and/or hasn't been properly maintained.  If the valve seat is scored, grooved or pitted, or if the valve face is in poor condition then low RPM pressure will be affected.  Lapping of these surfaces to remove such imperfections can markedly improve low RPM pressure output, but this of course can only be done by an approved maintenance workshop.

Other contributors may be worn bearings or rotor journals.  If there is even a small amount of play between either bearing or their respective journals, the rotor may ‘cock’ radially.  Wearing of the faces of the bearings may allow the rotor to ‘float’ axially.  This in turn upsets the close tolerances required for satisfactory pressure output as each of these gaps contributes a loss to overall performance.

There are many different possible causes of low pressure at low RPM, the important thing to remember is that winding up the relief valve will not correctly rectify the situation.  If your pump has 1000+100 hours T.S.O. (sometimes less) and is exhibiting the aforementioned symptoms, it is highly recommended that the pump be removed and sent for inspection/repair.  In some cases the repair may simply involve cleaning of the relief valve seat to remove a deposit.  Some gentle reworking of the relief valve/seat surfaces can also improve pump performance.  In many cases the bearings will be worn and require replacement and at worst the pump drive and/or rotor splines may be U/S in which case it is often cheaper to simply exchange your pump for an overhauled unit.

Some quick fix methods to try before removing a fuel pump are:

• Check for any obstructions in strainers/lines etc first and confirming any gauges for accuracy

• Disconnect the inlet line to the F.C.U., hold a clean, preferably white rag over the end of the hose and run the boost pump into a clean bucket.  Wearing suitable protective gear and while the boost pump is still running, cyclically block and unblock the end of the hose with your thumb.  This will blow the relief valve on and off its seat to try to dislodge any contaminant.

• Every now and then a pump is sent to us with reported low pressure only for us to find that the four relief valve housing screws are loose.  This has caused the pump to draw air from under the relief valve housing and all but kills the pump’s ability to produce any sort of pressure.  More worrying is the fact that there have been Airworthiness Directives out on this topic for quite some time.  The FAA currently lists AD2003-14-03 and CASA currently lists AD/LYC/110 amdt 1, both of which refer to the latest revision of Lycoming Service Bulletin 529(currently 529B) which essentially details regular checks for loose relief valve screws and what to do if you find them.

To keep your fuel pump performing at its best and above all else safe the usual common sense cliche’s are applicable:

• Keep the fuel tanks clean and maintained - cap seals, strainers etc.  

• Properly maintain all fuel system filters - both engine and airframe

• Lubricate the drive shaft upon installation in accordance with the latest revision of Lycoming S.I.1420A (failure to do this will severely degrade the life of your pump drive coupling and all but guarantee its replacement at overhaul if not earlier. The result of such a replacement will most likely render the pump as beyond economical repair [B.E.R.])

• Try some of our quick fix tips above first, if they don’t work it’s likely your pump is in need of repair or replacement

Crane Lear Romec piston engine driven fuel pumps are a dependable and proven component that will dutifully serve their purpose.  In our experience they are practically maintenance free until around 1000 hours.  The only maintenance they require is a clean fuel supply, lubrication of the drive shaft upon installation and at worst some fine tuning of fuel pressure following the post installation ground run.

Remember, the relief valve is generally bench set and adjusts pressure at high RPM not at idle.

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