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Twin Engine Airplane – Flying On One Engine

Twin Engine Airplane

It doesn’t take long to realize that most of the challenge of learning to fly a twin engine airplane is learning how to do it on one engine.  Compared to a single engine aircraft, the engines of a twin (with the exception of centerline thrust twins like the Cessna Skymaster) are not on the centerline of the aircraft.  Therefore, when only one engine is operating, the twin engine airplane is in an unbalanced, and potentially unstable state.  Here’s a picture of a skymaster — and if you learn to fly a twin engine airplane in one of these your license will be limited to centerline thrust configurations only, versus a twin engine airplane with conventional displacement of the engines out on the wings.

Twin Engine Airplane

twin engine rating

 

The Beechcraft travelair that I’ll be flying for this check ride does not have counter-rotating propellers.  Viewed from behind, both props turn to the right.  This is important because in a climb attitude, the descending prop has a larger angle of attack with respect to the relative wind than the ascending blade — taking a bigger bite of air — and therefore displacing more air towards the rear of the airplane.  This results in 4 main factors that become important when only one engine is turning:

  • The engine on the right has a greater lever arm about the Yaw Axis because the center of thrust is further from the centerline of the aircraft (P-Factor)
  • The accelerated slipstream created by engine on the right creates an area of increased lift over the wing that is also further from the centerline of the aircraft
  • The accelerated slipstream created by the engine on the right is further from the rudder so the rudder has less authority
  • The counter torque produced by the engine on the right adds to the rolling moment caused by the increased lift of the right wing

These four factors make the left engine the critical engine: IF THE LEFT ENGINE FAILS, YOU ARE IN A MORE CRITICAL SITUATION THAN IF THE RIGHT ENGINE FAILS, because the airplane is more unstable and difficult to control.

Lets go through each of these points one by one, with a few illustrations.  First a review of p-factor.  The Wikipedia consensus statement on P-factor reads something like this: When an aircraft is in straight and level flight at cruise speed, the propeller disc will be perpendicular to the airflow vector. As airspeed decreases and wing angle of attack increases, the engines will begin to point up and airflow will meet the propeller disc at an increasing angle, such that descending blade will have a greater angle of attack and relative wind velocity and therefore increased thrust, while the ascending blade will have a reduced angle of attack and relative wind velocity and therefore decreased thrust. This asymmetry in thrust displaces the center of thrust of the propeller disc towards the blade with increased thrust, as if the engine had moved in or out along the wing.

You can click on all of these figures to enlarge them to read my notes.

P factor twin engine

Seen from above, the situation looks something like this….on the starboard engine, the descending blade creates an accelerated slipstream outboard of the engine.  The increased velocity of the airflow over the wing creates a localized area of increased lift that wants to lift the right wing.  Also, in a conventional centerline vertical stabilizer and rudder configuration, this accelerated airflow is further from the rudder, decreasing its authority.  Ever wondered why airplanes are designed with multiple vertical stabilizers right behind the engines (like the Lockheed Electra in this illustrations) — Now you Know.

twin engine accelerated slip streamHere is a picture of a typical twin engine airplane with a centerline vertical stabilizer and rudder.  In this example you can see that the accelerated slip stream is 3 to 4 times further from the centerline of the aircraft on the right side than it is on the left side.  This has a significant effect on rudder authority at slow speeds when the relative proportion of airflow over the flight control surfaces produced by the wash from the propeller increases.

centerline rudder adverse yaw

Torque Roll.  If you remember back to your high school physics class on Newtonian motion, you’ll recall that for every action there is an equal and opposite reaction.  When a combustion engine turns a propeller in a right hand direction, the propeller tries to turn the aircraft in a left hand direction.  On the right side of the aircraft the torque effect increases the rolling tendency to the left since it reinforces the rolling tendency created by the asymmetric lift produced by the right wing.  On the left side (when the right side engine is inoperative) the torque also tries to roll the aircraft to the left, but this is counterbalanced by the right rolling tendency created by the accelerated slip stream over the left wing.

multi engine trainingSo here is an illustration of our one engine out scenario.  The Critical left engine has failed and the aircraft is yawing to the left and rolling to the left.  The accelerated slip stream is far from the centerline of the aircraft so the vertical stabilizer and rudder have less authority.

twin engine adverse yaw

Seen from behind the situation looks like this.  The left engine is out, the aircraft is banked to the left and yawing to the left as well.

Corrective maneuvers

The moves that we make to correct this situation do two things:

1.  They set the aircraft up for optimal performance.

2. They help identify the dead engine.

The flight attitude that we are trying to achieve is a “zero-side slip” attitude.  If the engine out attitude is corrected only with rudder inputs, the aircraft will slip towards the inoperative engine, decreasing climb performance.  This will occur even if the ball is centered on the turn coordinator.  The result of wings level flight with an inoperative engine is described as a moderate sideslip towards the inoperative engine. Climb performance will be reduced and Vmc will be significantly higher than published as there is no horizontal component of lift available to help the rudder combat asymmetrical thrust.

If the engine out attitude is corrected only with aileron inputs, there will be a significant bank towards the good engine, resulting in a large sideslip towards the operating engine.  This sideslip will also decrease climb performance and increase Vmc.  The solution is a combination of rudder and ailerons.  2-3 degrees of bank towards the good engine and rudder inputs to center the ball with the reference line on the side of the operative engine.  The exact combination of rudder and aileron input varies from model to model and also with flap and gear settings and with airspeed.  Unfortunately, there is no instrument inside the cockpit to tell when you have achieved a zero side slip configuration, but one can be made by taping a yaw string to the windshield (before the flight!) for training purposes.

zero side slipzero side slip

Zero side slip configuration.  2-3 degrees of bank towards the operative engine, rudder inputs to center the ball with the reference line on the operative side.  If you’ve got any time in gliders you are probably familiar with a yaw string.  It is basically the only way to tell directly if the airflow is flowing along the longitudinal axis of the airplane and therefore minimizing drag.  Here’s a picture of a centered yaw string, and surprisingly enough, a picture of a yaw string on the nose of an F14 that I found on the web.  Who would have thought that the pro’s would need a piece of yarn to help them with out?

yaw string glider  yaw string F14

I said earlier that our corrective maneuvers would also help us identify the dead engine.  The mantra goes something like: “Dead Foot, Dead Engine”, or “Idle Foot, Idle Engine”.  Because we are concentrating on flying the aircraft, our rudder input to stop the yaw will occur on the same side as the operative engine.  Left engine out requires right rudder, right engine out requires left rudder.  You get the idea.  Once we’ve figured out, and called out, “Left Foot Idle, Left Engine Out”, we can go through our checklist for dealing with an engine failure.

Sheble has their checklist organized into a 3 step process: POWER UP, CLEAN UP, and FEATHER.

First POWERUP.

If this engine is going to fail, it is probably going to do it when we’ve just rotated on a short field and are slow and heavy.  We’re going to go to Mixtures RICH, FULL PITCH on both props, and FULL POWER on both engines.  Now that we are at a maximum performance power setting, we’ll clean the airplane up for minimal drag and best climb performance.

Next CLEANUP

BOOST PUMPS ON, FLAPS UP, GEAR UP.  Okay, so far so good.  Airplane is clean and flying.  We’ll pitch for the blue line in order to achieve best climb single engine operative.

Finally, FEATHER

The last thing that we can do is minimize the drag caused by the windmilling propeller.  Remember that a windmilling propeller has the flat portion of the blade perpendicular to the airflow which creates a lot of drag.  We want to rotate the prop blades such that they are parallel to the airflow, minimizing drag.  The most important thing about this maneuver is to do it on the INOPERATIVE ENGINE.  It is super embarrasing to have an engine failure and then shut down the good engine by mistake.  The three step process for feathering is to Identify, Verify, and Feather.  Once again, we’ll want to call out, “left foot dead, left engine out”, but we’ll verify that by pulling back on the left throttle control.  If performance suddenly tanks, we’ve probably pulled back on the wrong throttle control.  But if there is no change to the way the airplane feels and flys, we’ll pull the prop control back to the feather position on the dead engine.

The checklist goes something like this (and can be downloaded in pdf format by clicking here) — CHECKLISTS

POWERUP — FLY the airplane
MIXTURES RICH
PROP CONTROLS FULL
THROTTLES FULL

CLEANUP — FLY the airplane
BOOST PUMPS ON
FLAPS UP
GEAR UP

FEATHER — FLY the airplane at the blue line
IDENTIFY (dead foot — dead engine)
VERIFY (pull back on dead throttle)
FEATHER (pull back on dead prop control)

Twin Engine Airplane – Flying On One Engine Questions?

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2 comments

  1. nichm

    thanks for the detailed info, was searching for it,

    wat abt the jet engines

  2. bob c

    It’s all good stuff :)
    I use this procedure: Good, Good, Good ……mixture rich, prop’s full forward, throttles full forward
    Gear and Flaps…………where do you want them
    Identify ………………….Dead foot dead engine
    Bad, Bad, Bad…………..Dead engine engine throttle idle
    Dead engine prop control to feather (or feather button push)
    Dead mix to cut-off
    Engine out check list

    No cross feeding unless you have altitude and on a million mile x-c ….. 12 minutes to get around the pattern is only 4.8 gallons of fuel (28.8 LBS) in a Cessna 320 skynight . Also the engine might have quit because of bad fuel. Ya think you want to x-feed bad fuel to a good engine close to the ground?

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