How do I access the fuel transmitter on my 19/23/24 airframe?
Search strings: Fuel sender, fuel sensor, fuel level transmitter
Technical Editor:
When evidence of a fuel stain appears in the area of the lower wing root seal, a transmitter leak is by far the most common culprit. True fuel tank leaks are very rare on the 19/23/24/76/77, due to the robust tank construction, and are almost always the result of an extremely hard landing or severe off-runway excursion.
– Fuel tank and sender work is A&P territory. Like many other instructions available in BAC, this information is intended to help you educate your technician, thereby reducing your maintenance costs.
– BE VERY CAREFUL while working with the fuel tank. Even when completely drained, the remaining fumes are usually very explosive. Efforts to ventilate the tank may simply make matters worse, as the vapors from remaining unusable/un-drainable fuel may simply get diluted enough to become more explosive. You must religiously guard against sparks; and leave openings in the tank for as brief a period as possible.
– On the 19/23/24, the sending unit is accessible via an access cover, which is behind the under-door upholstery trim piece. While the earlier aircraft have only one door, the sender access cover is in the same physical location on both sides.
– The access covers are structural components of the fuselage box structure. The door frame openings mandate a relatively rigid design in this area, which must be preserved. Make sure any damaged screws are replaced with the correct AN525-832R6 Cadmium-plated structural washer-head screws (not the common stainless steel truss-head panel screws). You can get these screws from Aircraft Spruce at a very modest cost. Make sure you have some spares on hand; I recommend replacing all of them. If still original, these screws will be very tight, and some will get damaged during removal. Frozen or damaged screws can usually re removed by cutting a slot across the head with a Dremel cut-off wheel. A large common (flat-blade) screwdriver, tipped with valve-grinding compound (for a better ‘bite’) can then be used to remove the screw. Don’t reinstall any damaged screws, or you’ll regret it later.
A size 8-32 ‘plug’ or ‘taper’ style hand tap can be run part-way into each screw hole, to clean up overly-tight threads. The self-locking anchor nuts were originally coated with a dry lubricant, which wears off after a couple of screw removals. The steel screws then tend to seize up in the steel anchor nuts, unless the threads are cleaned up a bit. Sears-Craftsman sells a convenient hand-tap tool that eases this chore. It looks like a screwdriver handle with a tap chuck on the end, and a hollow handle. It comes with 6-32, 8-32, and 10-24 taps. A 10-32 tap can be purchased separately and kept in the handle as well. Sears calls this handy tool a Tap Driver Set, item number 00952171000.
– With the access cover off, you will see a round fuel tank access plate, with the smaller round sending unit mounted in the middle of it. In theory, you can remove the sender nuts and remove the sender, without removing the big plate. Sometimes it doesn’t work out that way, because the bolts turn in their holes; and you have to pull the access plate. Or you may find the sender’s bolt heads on the outside (my personal preference; see below).
– While the access plate bolts are easily removed, the plate itself can be very difficult to dislodge from the tank rib. BE VERY CAREFUL! The tank rib and access plate are aluminum, and can be easily scarred. The plate often seizes to the gasket. It may take a lot of patient persuasion using a hair dryer, 3M adhesive remover, and plastic or phenolic putty knives/scrapers to get the access plate off the tank rib (and to clean up the old gasket). Make certain none of the old gasket material falls into the tank.
– Original access plate and sender gaskets were dry cork; some replacements are rubberized gasket material. Either type is acceptable. New gaskets can be sprayed with a product such as Perfect Seal gasket adhesive. DO NOT USE ANY KIND OF SILICONE! The intent of the adhesive is to make later gasket removal much easier, in addition to sealing small surface imperfections. Make sure that your product of choice advertises easy later gasket removal. These gaskets are still available; but the access plate gasket in particular has become ridiculously expensive ($118 in August of 2009). Gasket material specifications are shown below; they can easily be fabricated if necessary.
– The 800-001-10 sending unit bolt seals are essentially a rubber o-ring that is captured on the ID of a washer (Stat-o-Seal or Lock-o-Seal). In such small sizes, they seal much better when used on a flat surface rather than in the threaded area of a fastener. For that reason, if the seal is placed on the bolt, and the bolt then placed through the sender and plate, it will provide the best seal. If desired, you can use a second seal under the nut on the back of the plate (they are only about a buck each). Note that the nuts into which the access plate bolts go are sealed on the back. Additional seals are not normally required on those bolts.
– Sometimes a sender will leak at the center post. While a repair can be attempted there, in most cases a sender overhaul or new sender will be required.
– There is a lot of info on BAC regarding bonding the transmitters to the airframe. Make sure you read it, and do it all properly, before covering everything back up.
Here is the parts list for the required consumables:
35-921503 Transmitter gasket
800-001-10 Transmitter bolt seals; bolts are AN3
NAS1523-3R Superseding P/N for the 800-001-10 Stat-O-Seal
169-110010-9 Transmitter plate gasket with eight bolt holes
Gasket specifications are 1/16” sheet, synthetic rubber/cork, Class 1 grade medium, Mil-G-6183.