Complex Signoff

By , 28 April 2010 09:41

Most of the information on this page will assume the aircraft you’re receiving training in is a PA28R-200 (Piper Arrow). Why you ask? Because that’s what I learned on and that’s what I will be teaching in.  I only deal with the constant speed propeller portion of the complex aircraft.  For information on the retractable gear portion, ask your CFI and read your POH.

Definition: (14 CFR 61.31(e))

A complex aircraft is an aircraft equipped with “…retractable landing gear, flaps, and a controllable pitch propeller; or in the case of a seaplane, flaps and a controllable pitch propeller.”

Requirements for sign-off: (14 CFR 61.31(e))

You must have “(i) Received and logged ground and flight training from an authorized instructor in a complex airplane, or in a flight simulator or flight training device that is representative of a complex airplane, and [have] been found proficient in the operation and systems of the airplane; and (ii) [have] received a one-time endorsement in the pilot’s logbook from an authorized instructor who certifies the person is proficient to operate a complex airplane.”

Why we care:

When flying an aircraft with a fixed pitch prop, the prop is only at its most efficient at a single combination of RPM and airspeed. There are different props available that are most efficient in different phases of flight. Some are “climb props” while others are “cruise props.” These are engineered to be most efficient in the phase of flight for which they are named.

In a complex aircraft with its adjustable pitch prop, there is a prop for every phase of flight. The addition of the PROP control and the Manifold Pressure gauge are the most prominent differences in the cockpit. When the prop control is all the way forward, the prop is taking the smallest “bite” of air that it can as it spins. More technically, at this setting, the prop has the lowest angle of attack (all other things being equal). As the prop control is moved aft, the angle of attack of the prop increases and the prop takes an ever bigger bite of air.

With that understood, its time to change an old habit. When your instructor tells you to increase the RPM from 2400RPM to 2500RPM, you aren’t going to reach for the throttle anymore. In a complex aircraft, the throttle controls manifold absolute pressure (MAP), not RPM. In order to change the RPM, you must move the prop control.

MAP is a measure of the amount of fuel/air mixture flowing through the engine. It measures the absolute pressure of the mixture inside the intake manifold and displays it in inches of Mercury (in.Hg). When the engine is off, the manifold is at ambient air pressure and the MAP gauge will read accordingly (i.e. 29.97). Once the engine is started in a normally aspirated engine (i.e. no turbo- or supercharger) MAP will fall to some lower pressure. Idle may be around 12 inches. It is important to note that in a normally aspirated engine, MAP can never exceed ambient air pressure. Thus, if the engine fails in flight, you will actually see the MAP rise.

At a basic level, the prop control is how fast you want the prop to try and spin, and the throttle is how hard you want the engine to try and reach that speed.

For every RPM setting there is a maximum MAP. Using MAPs above this value will result in excessive engine wear and/or damage. In order to avoid this condition, you should follow a simple rule: “PROP on top.”

If you are increasing RPM and MAP, adjust the RPM up to its new setting first, then increase MAP.

If you are decreasing RPM and MAP, adjust the MAP down to its new setting first, then decrease RPM.

Let’s say you’re climbing out of the airport and have the engine set to 25″ and 2500RPM. As you level out, you want to change to a cruise configuration of 21″ and 2400RPM. Which lever do you move first?

First, pull the throttle back until the MAP reads 21″. Then slowly adjust the prop lever to reduce the engine from 2500RPM to 2400RPM. Done!

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