Airplanes can SKID!?!

Subtitle: What brought N207FD out of the sky?

We belong to a couple of safety groups that assist us in thinking through the safety guidelines of our program. One of the programs is MSI, from TN. They have researched many accidents, similar to ours, and have started to draw some conclusions.

As you read the article, keep in mind that our fuel level was similar to one scenario that they describe. Our left tank was down to the "unusable" fuel level and our right tank had about an hours' worth of fuel (15-16 gallons). Our fuel selector was on the right as well.

June 2010

A number of recent events have reminded us of an all too common danger when flying the Cessna 200 series aircraft. Hopefully everyone piloting one of these aircraft (Cessna 205, 206, 207, 210) is well aware that you must be very careful to switch tanks in a very disciplined and deliberate manner, otherwise there is a great risk of running a tank dry and the engine stopping at a very inopportune time. This can be very embarrassing (or much worse!), especially if you still have plenty of fuel in the other tank that was not selected. In most cases, switching tanks and hitting the electric auxiliary boost pump switch brings the engine back to life very quickly, but there are some exceptions, which we will discuss later in the article.

However, there is a much less commonly recognized danger relating to fuel supply and starvation in these aircraft, and that is called “unporting” of the fuel due to uncoordinated flight maneuvers. In the Cessna 200 series, the tubes “picking up” the fuel extend into the wing tanks from the inboard ribs. This is also where the fuel quantity transmitters are located in the 205, 206, and 207.

Due to the lack of much baffling inside the fuel tanks, uncoordinated flight, such as is experienced while slipping or skidding, can cause the fuel to move away from the inboard rib, and allow air to be drawn into the pickup tubes. The U206 is especially prone to this during photo shoots with the cargo door removed, as the photographer often asks for skidding right turns to allow him to get photos out of the open right side of the aircraft while keeping the right wing up with opposite aileron. If the left tank is selected while doing this, it is very possible to “unport” the pickup tubes in the left tank, even with half fuel capacity. Couple that with the typical request to be down low for good photos, and you have a disaster in the making.

Other examples of uncoordinated maneuvers that have caused unporting events are slipping in a crosswind on long final, extended cruise or descents using only rudder, climbing right turns after a low pass with low fuel in the selected tank, circling over objects on the ground, and tipping the plane to observe air drops.

What is the solution?

1. Always stay coordinated while maneuvering the aircraft. Your fuel is doing whatever the ball in the turn coordinator is doing.
2. Always have the fuller tank selected while near the ground.
3. Avoid the temptation to run your tanks down to the last 5 or 10 gallons. If you want to know why, look inside the tank next time it is that low. The fuel is barely an inch deep, and the pickup tubes are about half an inch off the bottom of the tank.
4. Finally, “just say no” to the photographer when they ask for a skidding right turn, especially close to the ground. Offer to climb to a higher altitude that will allow for gentle banking to keep the subject in sight without getting the wing in the way. Hopefully you briefed him ahead of time to bring a telephoto lens or two.

Regarding restarting the engine if unporting or fuel starvation occurs:

Testing has demonstrated that switching tanks and immediately hitting the emergency setting on the aux pump switch for a few seconds or until you see the fuel pressure or flow start to come back up brings the engine back to life the fastest. Waiting to switch tanks only delays the restart. Holding the pump switch in emergency position for longer than a few seconds can easily flood the engine, especially turbocharged ones or the newer IO550’s.

Another consideration affecting the restart has to do with maintenance. If the TCM service bulletin (SEB 96-4R1) regarding setting the resistors that control the pump speed with varying throttle positions is not followed properly, particularly in using a properly regulated voltage of 14 or 28 VDC, there is added danger of flooding the engine while attempting a restart. Some places have used the aircraft battery while setting the resistors, but that gives an incorrect setting, as the battery gives a lower voltage than the running alternator will.

As a final warning, unporting can be deadly, and has been already. Additionally, flooding the engine while attempting a restart, while not conclusively proven, is strongly suspected to have been a major factor in at least 3 recent fatal accidents in missionary aviation.