multi engine

Why do pilots fly a twin-engine airplane?

For safety. If one engine fails, the other engine can keep you flying.

Is speed, range, or payload the main reason for a twin?

No. Twins often do those things well, but a single with a bigger engine could do them more simply. The real point is having a backup engine.

What kinds of flying are twins best suited for?

Flights over water or rough remote land (mountains, deserts), medevac and high-priority cargo, and any mission where risk must be kept as low as possible.

What is the biggest downside when a single-engine airplane loses its engine?

You can't hold altitude and a restart isn't likely, so you have to make a forced landing.

What is the upside when a single-engine airplane loses its engine?

It flies the same with or without the engine, and the response is simple: restart checklist, then emergency landing checklist. No special training needed.

What is the upside of losing one engine in a twin?

You still have another engine, so you can usually hold altitude.

What is the downside of losing one engine in a twin?

The airplane pulls to one side, your workload goes up, and you need special training to handle it.

What is asymmetric thrust?

Thrust coming from only one side of the airplane. The working engine sits off to the side of the center of gravity (CG), so it tries to twist the airplane toward the dead engine.

After one engine fails, how much power do you lose, and how much climb performance?

About 50% of your power, but about 80–90% of your climb performance.

What is "excess thrust" (or excess power)?

The extra power you have beyond what level flight needs. You use it to climb.

Why does losing 50% of power cost about 80–90% of your climb?

Climbing only uses the small gap between the power you have and the power level flight needs. Cutting your power in half wipes out most of that gap.

What causes the extra climb loss after an engine failure?

Less thrust available, the dead engine being extra weight that makes no power, and drag from a propeller that is left windmilling.

What is a windmilling propeller?

A propeller on a dead engine that is spinning freely in the wind. It acts like a big dinner plate and creates a lot of drag.

Rank the drag from a dead engine's propeller: windmilling, stopped but not feathered, feathered

Windmilling is the most drag, stopped-not-feathered is next, and feathered is the least.

What does the power-required curve show?

How much power it takes to hold level flight at each airspeed. It is shaped like a U.

Why does flying very slow take a lot of power?

You need a high angle of attack to make enough lift, and that creates a lot of induced drag (drag from making lift).

Why does flying very fast take a lot of power?

Parasitic drag (drag from pushing the airplane through the air) gets bigger as speed goes up.

Where is Vy (best rate of climb) on the power curve?

Where the gap between the power you have and the power you need is the biggest. That is where you have the most extra power to climb.

What speed sits at the bottom of the power-required curve?

The speed that needs the least power. The instructor calls it best glide (Vg). Strictly it is best endurance speed, which is a bit slower.

What happens if you fly with one engine out, wings level, using only rudder?

The airplane flies in a "crab," with the nose pointed away from where it is actually going. That makes extra drag and wastes performance.

What is zero sideslip?

A way to fly with one engine out: bank 0–5° toward the working engine and use rudder to keep the ball centered. The airplane flies straight and lined up with the wind, so it makes less drag.

Which way do you bank in zero sideslip?

Toward the working engine. This is called "raising the dead engine."

Why does banking toward the working engine help?

Part of the wings' lift tips sideways and pushes against the sideways pull of the asymmetric thrust. You need less rudder and make less drag.

How much bank do you use in zero sideslip, and why that limit?

0 to 5°. More bank costs you lift (climb) and risks a stall.

How much does each degree of bank lower Vmc?

About 3 knots per degree, up to the 5° limit.

What do ailerons and rudder each do in one-engine-out flight?

Ailerons only set the 0–5° of bank, then stay neutral. Rudder controls your heading and keeps the airplane going straight.

What do you look for on the G1000 for zero sideslip?

Keep the bank pointer between the center diamond and the first 5° tick, and use rudder to center the slip/skid indicator (the "brick").

What does "split the ball" mean?

Use rudder to keep the slip/skid indicator (the ball or "brick") centered.

What should you do first when an engine fails?

Take a breath and don't slam the rudder. Notice which way the nose drifts, use gentle control inputs to stop the drift, center the ball, then start the engine-out flow.

Why shouldn't you react to an engine failure with big, fast inputs?

You can step on the wrong rudder (a 50/50 guess), roll into the dead engine (up to about 30° of bank), and end up uncoordinated and panicked.

What is the engine-out flow, in order? (instructor's version)

Pitch for 88 knots, mixtures, props, throttles, gear, flaps, identify the dead engine, verify, choose to feather. Then set up zero sideslip.

Below what altitude must a simulated engine failure use throttle only?

3,000 ft AGL. Below that, no mixture cuts or other methods. Above it, those are allowed.

Why is a spin so dangerous in a twin?

The heavy engines sit far from the CG and build up a lot of spinning momentum. Rudder can't stop it once it gets going. No light twin is approved for spins.

How could an engine failure lead to a spin?

You slow down, lose directional control, stall, and then spin.

What is mass moment of inertia?

How much of an object's weight is far from its center. The farther the weight, the harder it is to start or stop the object rotating. Twins have engines out on the wings, so theirs is high.

What is the critical engine?

The engine whose failure hurts the most. It makes directional control and climb performance the worst.

What is a conventional twin?

Both propellers turn the same way (clockwise as seen from the cockpit). The left engine is the critical engine.

What is a counter-rotating (non-conventional) twin?

The two propellers turn in opposite directions, inward toward the fuselage. It has no critical engine. The Piper Seminole is this type.

Why are conventional twins more common?

They are cheaper to build: two identical engines and one production line.

Why doesn't the FAA require counter-rotating props?

Engine failures are rare and a trained pilot can handle them. Counter-rotating props mostly show up on trainers, where engines get shut down all the time.

What are the four reasons the left engine is critical?

P-factor, accelerated slipstream, spiraling slipstream, and torque.

What is a memory trick for the four factors?

Yaw, roll, yaw, roll. P-factor = yaw, accelerated slipstream = roll, spiraling slipstream = yaw, torque = roll.

What is P-factor?

With the nose up, the propeller blade moving downward has a higher angle of attack and makes more thrust than the blade moving upward.

What is a moment arm?

The distance between a force and the CG. A longer distance gives the same force more twisting power.

How does P-factor make the left engine critical?

In a conventional twin the downward-moving blade is on the right side of each propeller, so the thrust shifts toward the right. The right engine pushes from farther out from the CG, so it has more power to twist the airplane when the left engine fails.

What is accelerated slipstream?

Each propeller blows extra air over the wing. The right side of the prop blows more, so more lift is made farther out on the right wing. If the left engine fails, the right wing has more power to roll you to the left.

What is spiraling slipstream?

Air blown back by the propellers corkscrews backward and to the right, onto the tail. When the left engine is the one working, its airflow on the tail helps cancel the yaw. The right engine's airflow doesn't give that help.

Why does losing the left engine hurt for spiraling slipstream, but losing the right doesn't?

The left engine was the one giving the helpful airflow on the tail. The right engine wasn't helping, so losing it costs you nothing.

What is torque (as a critical-engine factor)?

A propeller turning clockwise makes the airplane want to roll the other way (to the left). It is Newton's third law: every action has an equal and opposite reaction.

How does torque differ between a left-engine failure and a right-engine failure?

Left engine fails: the right engine's thrust and torque both pull you left, so they add together (worse). Right engine fails: the left engine's thrust pulls you right but its torque rolls you left, so they partly cancel (better).

What happens when the critical (left) engine fails on a conventional twin?

All four effects work against you at once: P-factor, accelerated slipstream, no spiraling slipstream help, and torque.

Why doesn't the Seminole have a critical engine?

Its counter-rotating props make these effects match on both sides, so losing either engine is about equally hard.

What is blue line?

Vyse, the best single-engine rate of climb speed. It is 88 knots in the Seminole, and it is the most important speed to fly after an engine failure.

What is red line?

Vmc, the minimum control airspeed with the critical engine out. Below it, you can lose directional control.

After an engine failure, do you fly blue line or red line?

Blue line, always. Stay far away from red line.

What is Vxse?

The best single-engine angle of climb speed (the most altitude gained for the distance flown). It is 82 knots in the Seminole.

What is Vsse and why does it matter?

Safe single-engine speed, 82 knots in the Seminole. Instructors won't simulate an engine failure below it.

What are Vle and Vlo?

Gear extended speed and gear operating speed. Both are 140 knots in the Seminole.

What is the gear retraction speed in the Seminole?

109 knots.

Which changes with conditions, Vmc or blue line?

Vmc changes with conditions. Blue line stays fixed because it comes from the airplane's design.

Where are the Vmc test rules defined?

14 CFR 23.149. It was replaced by 23.2135(c) after Part 23 was rewritten around 2016.

What is Part 23 versus Part 25?

Part 23 sets certification standards for light airplanes, including light twins. Part 25 is for transport-category airplanes like a 737.

Which Vmc rule applies to the Seminole?

23.149, because an airplane follows the rules in effect when it was certified.

Why does 23.149 exist?

To make manufacturers test Vmc under worst-case conditions, so the published Vmc is never worse in real life.

Why does Vmc change but stall speed mostly doesn't?

Vmc depends on weight, altitude, temperature, CG, configuration, and engine power. Stall speed stays about the same.

How does a windmilling prop on the dead critical engine affect Vmc?

It raises Vmc. The extra drag adds to the yaw, so you need more speed and more airflow over the rudder to keep control.

Does a low Vmc mean you can fly slower after an engine failure?

No. Slowing toward Vmc takes you away from blue line and closer to the ground. Stay at blue line.

How does banking up to 5° affect Vmc?

It lowers Vmc. Less drag and the bank pushing against the yaw give you more control authority.

How does a standard day (sea level, 15°C) affect Vmc compared with a hot, high day?

Vmc is higher. Dense air lets the working engine make more power, which means a bigger yaw to fight.

How does trimming for takeoff affect Vmc?

It doesn't really. Neutral trim just keeps the test the same for every airplane.

How does being out of ground effect affect Vmc?

It raises Vmc. You make more induced drag, so you have more to fight.

How does full power on the working engine affect Vmc?

It raises Vmc. Performance goes up, but so does the yaw from asymmetric thrust.

How do flaps in the takeoff position affect Vmc? (Seminole takeoff flaps = 0)

Retracting flaps cuts drag (performance up) but removes stabilizing drag, so Vmc goes up.

How does gear up affect Vmc?

Performance goes up, but Vmc goes up too. The gear hanging down acts like a keel that helps keep the airplane straight.

How does an aft CG affect Vmc?

Performance goes up (less tail down force needed), but Vmc goes up. The tail surfaces are closer to the CG, so they have a shorter lever and less control power.

Does more performance always mean a lower Vmc?

No. Ask what kind of performance changed. More engine power or less drag can raise performance and still raise Vmc.

Why are takeoff flaps 0 in the Seminole?

Flaps don't help you hold blue line. Keeping engine-failure risk low is more important than a shorter takeoff run. (Flaps 10 is allowed by the POH.)

With an obstacle ahead, what do you do with flaps on takeoff: single versus twin?

Single: use flaps 25 to climb over it. Twin: keep flaps 0 so you can hold blue line if an engine fails.

What is the priority order in multi-engine flying?

Lower the risk of an engine failure first, performance second.