We Get Questions

Sometimes it’s hard for experienced pilots to remember what it was like to be wide-eyed with the newness of flying airplanes. Often pilots and instructors get challenged with what sound like very basic questions from lower-time pilots … and if we’re smart we’ll treat these “simple” questions as educational opportunities for those joining our ranks, not irksome distractions cast aside like Jepp charts previous revision. Remember, one of these new pilots might be sitting to your left in the cockpit of a Regional Jet when you get laid off from your “major airline” or corporate job in the future <grin>.

So as education for newer pilots out there, as well as refresher for others (including myself, since I have to make sure I remember things correctly before responding), here are a couple questions that have come across my desktop recently, and my responses:

Q: What are unfeathering accumulators, and when do you use them?

What it does: Unfeathering accumulators are devices often found on multiengine airplanes, designed to allow easier engine restarts after a propeller has been stopped (“feathered“) in flight.

Where it is: Behind the engines, often aft of the firewalls, there is a container that is called the unfeathering accumulator. Inside the accumulator is a piston or diaphragm separating sections within the chamber – one side is connected to the engine’s oil supply through the propeller governor, the other contains air (usually nitrogen) under pressure, serviced through a filler on the back side of the accumulator.

How it works: In normal operation the propeller governor boosts oil pressure inside the accumulator, pushing against the nitrogen charge. When the propeller is feathered (as during a practice engine shutdown for training, or in the case of a real-world shutdown) oil is removed from the propeller dome, allowing the prop blades to twist enough that aerodynamic drag (in the direction of rotation) causes them to stop. A check valve in the propeller governor closes off the dedicated oil line, trapping high-pressure oil inside the accumulator.

When the propeller control is moved out of feather during engine restart, this check valve opens and routes high-pressure oil, propelled by the nitrogen charge against the piston or diaphragm, to the propeller dome. This (hopeful) twists the prop blades enough that they “bite” into the slipstream and are driven by wind force to begin rotation; if ignition is working and the fuel-air mixture supports combustion, the engine will restart.

Why it matters: Without accumulators, or if the accumulator charge is insufficient to push the propeller blades fully out of feather, the pilot will have to “bump the starter” to get the prop spinning – use the starter to rotate the propeller. Sometimes you have to put the airplane into a shallow dive, increasing speed and aerodynamic force on the propeller, to get a restart.

Maintenance: The nitrogen charge of the accumulators should be checked no less frequently than at annual inspection. Between annuals you can perform a rough check of the charge of the accumulators by performing both propeller feather tests simultaneously during your engine run-up-if one engine’s accumulator charge is low, or if there is a minor leak in the accumulator, that propeller will lag significantly behind the other in the speed with which it comes back “out” of the feather check.

Liner notes: Accumulators are only valuable if you intend to shut down an engine and restart it in flight. Outside of training, in almost all cases if you feather a prop “for real,” you won’t want to try to restart that engine before it’s on the ground and checked out. Although accumulators often look good on a “spec sheet” when selling the airplane, many pilots prefer not to have 10 pounds or more in useful load dedicated to carrying the weight of the accumulator hardware.

Q: The upper part (metal retainer plate) of the oil dipstick on my engine is rusting, symptomatic of regular condensation build-up inside the engine. I am particularly worried that this is releasing rust stains/particles in the oil. Do you recommend replacement of the rusting part with a carefully re-riveted stainless steel plate or should I simply buy a new dipstick?

Concerns: Yours is a common situation. In extreme cases, the dipstick bracket has rusted completely through, allowing the stick itself to drop into the engine oil sump…from where it is very difficult to remove.

Treating the symptoms: Either option (a new dipstick, or a stainless steel replacement) will eliminate the corrosion problem, at least in the short run.

Addressing the cause: This rusting comes from condensation that forms on the dipstick when moisture from hot engine oil is cooled where the dipstick is in contact with cooling air flow in the engine. There’s no way to prevent this condensation from forming–but you CAN prevent it from settling on the inside of the dipstick, where it will corrode the metal. Try drying the inside of the oil cap with a cloth, then spraying a liberal dose of LPS-1, WD-40 or some other moisture-displacing lubricant on the inside of the cap. These lubricants will prevent water formation on these “rust-able” surfaces.

Q: In a light twin, if I have rotated without enough remaining runway to land, the gear is still down and I lose an engine, what am I to do?

The problem: You’ve discovered why the procedure in light twins is almost always to retract landing gear as soon as you’ve confirmed a positive rate of climb (an exception is the Cessna Skymaster, whose gear retraction geometry significantly increases drag while the gear is in transit). Unless the airplane is VERY lightly loaded and the air temperature very cool, you will not have enough power to climb on a single engine with the landing gear down. Most light twins cannot even hold level altitude on one engine in this configuration, let alone climb.

In fact, the rate of deceleration on a single engine with gear extended, when begun from an initial climb attitude, is so great that almost all other light twins will rapidly slow to “red radial” (VMC) speed and below if the pilot attempts to continue a climb after engine failure.

The risks:But wait,” you might think, “the landing gear comes up rapidly. Can’t I hold it off long enough to immediately retract the landing gear?” In my experience, (having presented this precise scenario literally thousands of times while instructing in the Beech Baron simulator at FlightSafety) no. Even with practice (and knowing that the failure will occur at 100 feet above ground, with the gear still down), almost no one I ever trained could safely “fly out” of the scenario. Add the “surprise” factor and lack of immediate, prior practice, and I’m sure that virtually no one can avoid an incipient VMC event, followed by a wing strike and uncontrolled cartwheel, if attempting to continue to fly under such circumstances.

Most light twins’ POH performance charts confirm this, showing that the “Accelerate-Go” option is possible under the best of circumstances only at very light weights and low density altitudes.

Strategy: What I’ve always taught is “if the gear is up, go up; if the gear is down, go down.” By “go down” I mean immediately retard BOTH throttles and land straight ahead, under control, just as if you’d experienced an engine failure on takeoff in a single-engine airplane. Historically your chances of survival are much better if you do this than if you attempt to fly out of an engine failure in the takeoff configuration.

Retract the landing gear upon confirming a positive rate of climb in order to limit your exposure to this critical scenario in the unlikely event of an engine failure immediately after takeoff.

THE BOTTOM LINE: Questions are how new pilots, and pilots new to airplane types or flight operations, learn. Answering questions makes them better … and also refreshes and informs the person giving the answer. Got questions? Send me an e-mail and we’ll all learn.