Aircraft Dipsticks
General
Good fuel management is good airmanship. While good fuel management involved many aspects, one of the more important ones is having enough. So in this article I want to talk about establishing how much fuel we have in the aircraft’s tank(s).
Fuel loaded onto aircraft is measured in units of volume, whether the fuelling is carried out by the pilot or by ground personnel refuelling the aircraft for the crew. The volume of the fuel is measured in either litres, Imperial gallons or US gallons. However for operational reasons it is the weight (mass) of the fuel that is relevant to the pilot(s) and the weight of fuel is generally indicated in kilograms or pounds. The actual amount being determined by conversion from volume to weight (mass), at the appropriate specific gravity for the type of fuel. Clearly there is plenty of potential for serious error.
The New Zealand Civil Aviation Authority agrees with the ICAO Standards and Recommended Procedures (SARPS) that fuel weight (mass) be measured in kilograms and fuel quantity be measured in litres, although it is not intended that this be mandatory.
One of the mitigations against an error in fuel quantity management is the careful measuring and indication of fuel quantity. Some aircraft types, of all sizes and category, have very accurate and reliable fuel measuring systems. But unfortunately some do not.
In aircraft without accurate and reliable fuel measuring systems a much more conservative approach to flight planning and fuel management must be employed. The careful use of fuel dipsticks is often part of such conservative management. To avoid the sound of silence, fuel dipsticks should be used whenever an aircraft's integral fuel quantity measuring devices are limited, faulty or otherwise suspect.
The following information is derived from an old CAIC GEN (A78 - 30 September 1985) and relates to the fabrication, calibration, use and storage of dipsticks.
Fabrication of Dipsticks
Dipsticks should be fabricated from a non-magnetic material, finished so that it is unlikely to damage the fuel tanks or cells.
Many dipsticks are fabricated out of wood. It is generally better if the dipstick is not painted, however if it is painted, ensure that it is a type of paint that will not be affected by the fuel and the paint is unlikely to chip or flake. Ensure that the surface finish will not facilitate excessive creeping of the fuel level which may lead to over indication of the fuel quantity.
Many dipsticks are circular, which can be fine, however the use of a rectangular or square section may make it easier to mark and will more easily allow different units to be included.
The top end of the dipstick should be fabricated so that the dipstick physically cannot fall into the tank. If it can fall in it will, I promise! I have dropped a dipstick into an aircraft fuel tank only once but the tank needed to be removed from the wing by the engineers to extract it. As you can imagine I was not popular with the boss. If this is impractical, the dipstick should be fitted with a "streamer" to facilitate its removal from the tank.
Calibration of Dipsticks
All dipstick markings should be permanent, preferably engraved, and should include the aircraft’s registration letters and the units of measurement.
It may seem obvious advice but don't use another aircraft’s dipstick! While the aircraft may appear to be the same, it is quite possible that the same type of aircraft may have different shaped or sized fuel tanks.
In addition to the manufacturer's preferred markings, complementary scales in differing units of volume or weight may be added on separate surfaces of the dipstick, providing no chance of confusion exists.
If the aircraft has tanks with differing capacities, a separate dipstick should be used for each tank location, rather than to calibrate a common dipstick. Each dipstick should be clearly marked to indicate which tank it relates to.
The dipstick should be marked to indicate "empty" when the tank contains the unusable fuel quantity stated in the flight manual. In other words, the quantity indicated by the dipstick should be usable rather than total. This way, if a pilot further reduces the indicated quantity by the unusable amount in error, they will have erred on the safe side. However, if the dipstick were to have been fabricated to indicate total and the unusable were not subtracted from the indicated amount, then there would be less fuel available for safe flight than may be anticipated.
To establish the "empty" mark, the fuel system should be drained at its lowest drain point. Allowing for any undrainable fuel, add the quantity of fuel required to make up "unusable" fuel. With the aircraft in the attitude that it would be in when the dipstick is used, mark the dipstick "empty" at the fuel level indicated. Calibrate the remainder of the scale by adding known additional quantities of fuel and marking the dipstick accordingly.
Dipstick Use
For most aircraft the dipstick should be held vertically while making fuel quantity readings. However, some aircraft may have unusual filler neck geometry necessitating special techniques for which instructions should be supplied.
The dipstick should never be forced or allowed to drop onto the bottom of the tank, as damage may occur or paint flakes may be dislodged.
Dust and debris should be wiped off the dipstick with a clean cloth before its insertion into the fuel tank.
Stowage
The dipstick should be in either of two places only... in your hand or in the stowage receptacle. Not on the ground, as this can lead to contamination and not on the cowling, where it can be left to become FOD.
Stowage should preferably be in a specially made pocket so that the dipstick is protected from contamination and is readily accessible from the ground outside the aircraft. The stowage pocket should have suitable provision (e.g. a flap or straps) to retain the dipstick.
Before stowage, the dipstick should be wiped dry so that residual fuel vapour will not cause discomfort or danger to the aircraft occupants.
Summary
Dipsticks should be fabricated from material that facilitates accurate reading and is unlikely to damage the fuel tanks or contaminate the fuel. The top end of the dipstick should be fabricated so that it cannot fall into the tank, alternatively it should be fitted with a "streamer" to facilitate its removal from the tank because if it can fall into the tank, it will!
All dipstick markings should be permanent and should include the aircraft’s registration letters and the units of measurement. Never use another aircraft’s dipstick!
The dipstick should indicate usable fuel quantity.
The dipstick should be positioned into the tank appropriate to the aircraft type, usually vertically, so that an accurate reading of usable quantity is achieved. And be gentle with both the dipstick and the aircraft, and wiped off the dipstick with a clean cloth before its insertion into the fuel tank.
The dipstick should be either in your hand or wiped dry and in the stowage receptacle.
These small disciplines may be all that stands between you and a demonstration of how well you can glide the aircraft into a field - for real! I for one would rather not have to prove my ability, or lack thereof.