Floatplane Destabilization and Inspection
By Joel Wattum
Float season is almost upon us here in Alaska. I thought I would take the time to post a few ideas that will make it a safer season. We did a lot of flight testing for Aerocet floats to be added to 170 and 172 aircraft. What I am about to write about, seaplane stability, comes from our flight testing. I personally have thousands of hours flying many makes and models from Cessna series to DeHavilland Beavers for air taxis and lodges mainly out of the Kodiak area. The most recent 2023 Water Flying Special Edition has an article written by Mary McEnroe about acquiring her rating. One paragraph talks about the ball dancing around in the turn and bank indicator.
This is not unusual in many floatplanes. It was during flight testing we discovered one of many causes. We had the 172 series ready for FAA flight testing in late June. Due to scheduling it wasn’t going to be possible for testing by the FAA until October. The DER we were working with asked if maybe we could incorporate bow strakes or as commonly called, spray rails. We decided to give it a try which meant more flight testing. Walking around the float ponds we noticed several of the exact same make and model of aircraft on the exact floats with many different sizes and shapes of spray rails. From almost halfway back to the front spreader to all the way to the front spreader. Depth below the chine was usually 3 inches but many were even wider.
Researching IPC’s for floats NONE had spray rails. I conferred with the DER and also the owner of Aerocet. We came to the conclusion that the strakes did not need to be further aft than the face of the propeller since the bow wake would not reverse and go forward. We decided to try 3 inches below the chine. Temporarily installed the spray rails about 16 inches long and went flying. Water work showed dramatically less bow wake with the rails. In flight we lost all stability. I was back to the rudder pedal dance. A light came on and I realized all those floatplanes I’d been flying for decades were destabilized with the bow strakes or spray rails. If you see the photo of the Beechcraft 1900 It has strakes on the tail. They are not there for looks.
After several modifications we got very good bow wake with the strakes only 1.5″ below the chine and got our directional stability back in all 4 corners of the c.g. envelope.
Another facet of stability is that the fact that lots of float planes have a ventral. This adds to stability. It is also a dock banger. If you go from centerline outboard you can have a smaller strake. Beavers had a very large ventral and Kenmore STC’d the small ones further aft on the ends of the horizontal stabilizer. Those spray rails being outboard of centerline really add to the destabilization as anything aft of c.g. stabilizes the aircraft and anything forward destabilizes.
First off I am not advocating removing spray rails even though per CFR 43 Appendix A, anything done to a float or hull is a major alteration. I am advocating reducing rail size. If the photos went they show how much on many different aircraft we have removed. I am now down to 1.25″ and again no further aft than the propeller face. Every person I’ve modified this way notices a major difference in flight characteristics being much improved.
Oh one other fact. Most spray rails have a gap below the chine where the rails are riveted to usually angle. I could not find the video but with the water taxi there was spray going directly into the propeller between that gap. Like putting your thumb over a garden hose. We sealed it with 3M 4200. Problem solved but, I have heard many people tell me that they put spray rails on and it did nothing. If nothing else seal the gap.
While on the subject of the forward c.g. destabilizing an aircraft, I was giving a flight review to an individual who had just gotten his rating and the insurance wanted 3 hours training minimum. The aircraft was a Piper PA 14. We had installed it on floats and per the TCDS it was supposed to have a Piper ventral. Being pretty much unobtanium I knew of an STC by Roger Borer that allows the ventral to be removed and cable and springs are attached to the tail grab handles and the rudder.
We were flying and while doing air work we were doing the rudder pedal dance. I asked if we could try a skid test. He agreed and before I had rudder halfway in the inside wing stalled and the outside wing went over the top, the classic spin entry. I quickly recovered and asked if we could try again at a higher airspeed. We did and it spun horizontally like a top. We were keeping the wings level and had released the full rudder input. No way a ventral and the springs would help. I had already modified the spray rails. Looking at the STC and the TCDS the plane was originally certified on floats with a much smaller engine and propeller. This had a 180 h.p. Lycoming and 82 inch propeller. It is the propeller diameter that contributes to destabilization. Many aircraft obtain STC’s for engine propeller, wing etc. modifications. They are almost always for LANDPLANE only. It is up to the installer to determine if they are compatible with other STC’s or configurations. Without flight testing there is really no way to determine that. I admonished him to definitely fly higher airspeeds and not ever skid ever as it is usually landing at lower altitude and slower airspeeds that would get you in trouble.
Last summer we had 3 crashes in Alaska on 180series aircraft on Edo 2960 floats. Two were fatal and the third the occupants swam to shore. All were preventable if the floats had been inspected and bottom work done. Unfortunately, most planes get annualed on wheels, floats get put on in spring and taken off in fall with little or no attention. When your plane goes on floats it should be determined if the floats are in a safe condition.
Things to do is open all hatches and a thorough visual inspection inside and outside for condition and corrosion. Pump out hoses are often found laying in the bottom of the float or, the bottom retaining has corroded away. Often the hoses have a crack or other damage so they are not pumping water out from the bottom of the float. If they are harder than woodpecker lips it is time to change them. Water rudder hinge points and all cable attach points should be inspected by determining play as many fail when the hole elongates and gets paper thin at the edges Like the aircraft, floats should get a very thorough inspection annually.
