Maybe this is more normal in an aircraft with the really bad (1970s type) fuel gauges, but I’ve just seen this posted on a US site, by a TB20 owner:
Took my TB20 on first long trip since I bought it – from home in SC to San Antonio. On last leg I forgot to switch fuel tanks (in IMC and quite busy with Houston Center and San Antonio Approach, and engine sputtered when right tank went empty. Quickly realized the problem, switched to left tank, and engine recovered before quitting. Very lucky!! I should have realized the problem much earlier as the lateral controls were getting heavier and heavier. Could not get autopilot to maintain heading control. Real question is this- should the autopilot continue to follow the gpss steering with 0 fuel in right tank and a full left tank? Also, I landed in a gusty right crosswind with the left tank about 3/4 full. Control on final was difficult at best under those conditions. Should I have burned more petrol to about a quarter tank before landing?
It would appear that he started with 3/4 tanks and flew until one of them was empty.
Is there a certification requirement for an aircraft to be able to fly with one wing full and one empty?
There are sometimes limitations on autopilot installations. I don’t think these are torque related; the servos I have messed with on the bench have something like a 10000:1 geardown on the motor and have way too much torque to notice any extra torque on the roll axis. They may thus be related to the aircraft flying increasingly crooked in roll and yaw. On the TB there is a rudder trim but there may be an assumption that this is not being used.
On Cessnas with long range tanks, especially the C182, you do notice a fuel imbalance if it gets really bad, as apparently happened to the pilot you quote. For TB20 I wouldn’t know. Before anyone says ‘but Cessnas have crossfeed’, yes, they do, but most of them draw fuel very unevenly, at least for the first half of the tanks. This seems to stem from the venting of the tanks and the position of the vent tube, which, due to its position, gets bumped into a lot over the years. For some reason seems to be most prevalent in 182s.
I mostly fly a C210 without x-feed and my recipe to guard against disaster is to correlate the minute-hand of my (analog) watch with the position of the tank selector. IOW, if the minute hand is on the left hemisphere between 6 and 12, left tank, between 12 and 6 right tank. Has – so far – kept me out of the weeds!
Many types have fuel imbalance limitations. This will be related to aileron ability to lift the wing.
Most medium – old GA aircraft fuel gauges are useless.
The more recent Cirrus have excellent fuel gauges plus a two stage automated warnings for fuel imbalance. The first is an Amber Warning (IIRCC at 5 gals imbalance) and a Red Warning (IIRCC at 8 gals imbalance). You can’t cancel this Red Warning.
On my Cherokee 6, I have 4 tanks, two of which are out on the extreme wing tips, which can cause a really serious imbalance especially at low speed.
My “Best Buy” was a Fuel Totaliser that is accurate to about 0.2 of a Litre. My “system” to keep the a/c in balance is to alternate tank usage based on the fuel used. I keep a log showing the fuel in each tank at start up, the reduce each total by the amount shown as used on the totaliser.
As usual the answer is more complicated.
Someone dug out the following:
Don’t think it is a problem for most GA aircraft, probably Beech Aircrafts? some of their single piston and turboprops have fuel vs CG limits for pitch imbalance but I doubt anything serious for wing/roll?
I heard King Airs have strict fuel vs wing/roll imbalance and this has been attributed to few loss of control accidents in these twin tuboprops
Fuel imbalance may be more of an issue with twins than singles, as the wing tanks will be further outboard in twins, as the engines typically occupy the place where wing tanks would have been in the single. Though imbalance on a twin might be fine in normal circumstances, it could be troublesome if coupled with a failed engine on the inconvenient side. That said, twins more likely have aileron trim to overcome imbalance. Two twins I have flown, which I know have specified imbalance limits, are the DA-42 and Piper Cheyenne. I have purposefully tested the DA-42 exceeding the imbalance with no difficulty. The Cheyenne with an empty wing tip tank (fuelling error) was a misery, and the limitation appropriate. I flew the Cessna 310, 340, and Piper Aztec lots, and don’t recall any imbalance limitations, nor concerns. The Turbine DC-3 simply requires that outboard fuel be loaded symmetrically, what the pilot does after that is not limited.
Otherwise, for GA singles, I have never heard of an imbalance limitation, though most will warn you by roll force that they would like to be balanced.
The AFMS for the Osborne Tip Tanks of a Debonair says: „Load Tip Tank fuel symmetrically. If unsymmetrical fuel loading should occur in flight, adequate control is available for landing.“ The Debonair has an aileron trim.
Pilot_DAR wrote:
Otherwise, for GA singles, I have never heard of an imbalance limitation
The DA-40 with LR tanks spring to mind, the Cirrus SR22, Corvalis and the Extra 400.
Yes, all the modern ones. It seems that it is a change in certification requirements somewhere along the way.
