Al O’Donnell:
I looked at my maintenance manual but I cannot tell if my 1980 Sundowner wing tanks are really tanks or our wings are wet. Can anyone enlighten me. I saw the recent discussion, and up to then I always thought (for no good reason) that our tanks were units within the wings.
There can be a lot of misunderstanding on the 19/23/24 fuel tanks simply because of terminology. All these planes used “wet-wing tanks”. This means that they do not use fuel cell bladders, and they don’t have separate, readily-removable tanks. However, the tanks do consist of a separately-built tank section (with extremely strong end ribs), which is then riveted into place, to become an integral part of the wing that is designed to be able to hold fuel.
There are several characteristics that make the tanking system design quite unique in these planes:
1. They have heavy forged ribs that form the inboard and outboard tank bulkheads. This makes them quite strong compared to most designs.
2. They are inboard, next to the fuselage, which adds several advantages and one disadvantage. It makes the tanks even stronger, due to the heavy skin used there for the wing-walk and stress loading areas, and the stiffer structural elements within the wing. It greatly reduces tank flexing, compared to the more common mid-wing and outboard tanks. It significantly reduces roll trim effects due to fuel burn. The main drawback is having fuel next to the cabin in the event of a crash. The high strength of the cockpit and tanks helps mitigate this risk.
3. The Beech construction used bonded (structural aluminum epoxy) seams. There are relatively few riveted places on the wet side of the tanks. For this reason the Beech tanks (unlike Mooney et al) did not require a “sloshing sealant”. Ask anyone who has had to deal with aged tank sealants, and you’ll understand why this is such an advantage on the 19/23/24.
Actual tank leaks, as opposed to loose line fittings, leaking fuel level senders, bad sump drains, old hoses, and vent line problems, are very unusual in these planes. One thing to watch for, when buying one of these airframes, is evidence of slightly protruding fasteners above the main landing gear attach points. That is a dead giveaway of very hard landings, which can affect the long-term integrity of the fuel tank area in the wings.
As an interesting aside, one of Bob Steward’s favorite airplanes, the small Grummans, also had a tank system that was a unique design with some advantages (and drawbacks). A large tubular spar that also contained the fuel.