When the fuel flow transducer is in the horizontal position, the rotor within it are proper oriented and aligned with fuel flow.
This question comes up often enough so we decided to write on how best to install the fuel flow transducer model 201B manufactured by J.P. Instruments in an aircraft.
Orientation of the 201B fuel flow transducer:
To start off and assuming you have removed the existing fuel flow transducer, hold up the new 201B fuel flow transducer in a horizontal position with the three wires up – because this is exactly how it is to be fitted. Fitting the fuel flow transducer any other way will lead to its failure. When the fuel flow transducer is in the horizontal position, the rotor within it are proper oriented and aligned with fuel flow. This is how it is meant to operate. Also, while fitting it, be sure that there is no bend closer than 6 inches either in the inlet or outlet pipe i.e. no bends before or after the transducer. Also, do not use aluminium or brass fittings when installing the 201B fuel flow transducer – use only AN816-7 Steel. The transducer is engineered to seal tight so do not use Teflon tape, or other thread sealant compound of any kind. Also, the transducer is marked IN and OUT for the fuel lines – so make sure you attach it correctly or the rotor will malfunction. The maximum torque should not exceed 15 ft lb (180 lbs.) or roughly, two full turns past hand tight.
If your aircraft has a carburettor, the fuel flow transducer should ideally be at the same height as the carburettor. If it is higher, fit an anti-siphon loop in the line that peaks higher than the transducer. If your aircraft sports a fuel injected engine, fit the fuel flow transducer between the engine fuel pump and the servo regulator. If your aircraft has that unique vapor return line, install the fuel flow transducer between the servo regulator and the flow divider (spider).
We have seen aircraft with Aircraft Flow Sensors mounted between the electric boost pump and the mechanical pump – this is not recommended. Also, please do not hard mount the transducer to the engine as engine vibration can damage the internal rotor or cause it to malfunction and give misleading results. When installing the 201B fuel flow transducer, the wires are to be connected to the EDM. The fuel flow transducer gets its power supply (12V) directly from the EDM. Also, the wires should not be route along with or adjacent to ignition wires, alternator wires, spark plug leads, or cabin heater ignition wires.
When done, set the K-factor. The Fuel Flow Transducer 201B will have a K-factor of ~29.
It might be worthwhile to also procure a Fuel Flow Tester (just $35.00) for testing instrument and transducer.
Some pilots called in to report consistently high Aircraft Fuel Flow indicator readings. If you have followed above installation procedure correctly, check for air entering system at selector valve (may require a change of the O-ring). Here’s a test: The next time the aircraft is flown in cruise, turn on the boost pump for one minute. If the fuel flow drops, then this is an indication that air is leaking into the fuel line upstream of the transducer. It is possible that air is leaking into the system through the seal valve or other connection. Fuel stain may not be visible.