How to Determine the Critical Engine Using PAST (Multi-Engine Pilot Edition)

wifiCFI
Jan 1
5 min read

If you’ve done any multi training in a conventional twin, you’ve heard the phrase:

“Identify the critical engine.”

And if you’ve ever tried to explain why to someone who hasn’t taken multi yet, you’ve probably started drawing arrows, got halfway through “descending blade,” and watched their eyes glaze over.

Here’s a clean way to teach (and remember) it: PAST.

P-factor
Accelerated slipstream
Spiraling slipstream
Torque

These four asymmetric effects help you determine which engine’s failure creates the worst controllability and performance case—the definition of the critical engine.

Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >

First: What “Critical Engine” Means (In One Sentence)

The critical engine is the engine whose failure results in the most adverse effect on controllability and performance.

Not “the engine most likely to fail. ”Not “the engine on the left. ”It’s purely a worst-case engine-out handling/performance concept.

Also: in many counter-rotating twins, the critical engine is effectively eliminated because the asymmetric effects are designed to balance out. But PAST is still useful—because it teaches you what asymmetry is doing to the airplane.

Before You Apply PAST: Confirm You’re in a “Conventional Twin”

PAST matters most when both props rotate the same direction as seen from the cockpit (common: both clockwise). If that’s your airplane, there is usually a critical engine.

If the props counter-rotate, the classic “left is critical” result may not apply.

The PAST Acronym: The Four Effects That Drive the Critical Engine

P — P-factor

What it is: At high angle of attack and high power (takeoff/climb), the descending blade produces more thrust than the ascending blade, shifting the engine’s effective thrust line.

Why it matters: The thrust line moves sideways, changing the yawing moment arm (thrust × distance to centerline).

How it points to the critical engine: In the most common conventional twin (both props rotate clockwise from the cockpit), the descending blade is on the right side of each prop disk. That typically shifts each engine’s effective thrust line to the right.

On the right engine, “shift to the right” moves thrust farther outboard → bigger moment arm → more yaw if the left engine fails and you’re hanging on the right engine.
Bigger yaw demands more rudder, gets you closer to Vmc, and hurts climb.

So P-factor tends to make the left engine the critical engine in a conventional clockwise-rotating twin.

A — Accelerated slipstream

What it is: The prop blast increases airflow over parts of the wing and tail, improving lift and control effectiveness—but not uniformly.

Why it matters: More energized airflow over the wing behind the operating engine can change the rolling tendency and the “feel” of the airplane with an engine out.

How it points to the critical engine: Because the operating engine’s slipstream tends to energize its side of the airplane more, the “best case” is when that extra airflow helps you counteract yaw/roll. The “worst case” is when the energized side doesn’t help as much—or increases the tendency you’re already fighting.

In most conventional twins, accelerated slipstream generally adds to the asymmetry that already makes one engine-out case worse (commonly: left engine failure), even if it’s not the biggest contributor compared to P-factor.

Pilot takeaway: A magnifies the imbalance created by losing one engine.

S — Spiraling slipstream

What it is: The prop wash doesn’t go straight back—it spirals around the fuselage and hits the vertical tail at an angle.

Why it matters: That spiral airflow can either help or hurt your ability to counteract yaw by increasing (or reducing) rudder effectiveness.

How it points to the critical engine: In many conventional twins, the spiral slipstream from the operating engine strikes the tail in a way that can create a yawing tendency and/or affect rudder authority. Depending on the airframe and prop rotation, one engine’s slipstream can produce a tailflow interaction that is less favorable when the other engine fails.

This effect is more airplane-specific than P-factor, but in the common “both clockwise” setup, it typically doesn’t rescue you from the fact that one engine-out case has a larger yawing moment. It’s another reason the “left engine is critical” conclusion often holds.

Pilot takeaway: S affects how effective the tail is at fighting yaw when one engine is producing all the power.

T — Torque

What it is: Newton’s third law: the engine/prop rotates one way, the airplane tends to roll the other way.

Why it matters: In an engine-out scenario, torque from the operating engine becomes more noticeable because you’ve lost the balancing effect of the other engine.

How it points to the critical engine: Torque adds roll tendency that often couples with yaw and induced drag (because you’ll be using rudder and sometimes aileron to hold attitude). It typically worsens the overall “one engine-out case is harder” problem.

It’s usually not the primary reason one engine is critical, but it contributes to the workload and drag that reduce single-engine performance—again reinforcing why one side often feels “more demanding.”

Pilot takeaway: T piles on—more roll tendency, more corrections, more drag.

Putting PAST Together: A Simple “Critical Engine” Method

Here’s the pilot-friendly way to use PAST without turning it into a physics lecture:

Determine prop rotation direction (as seen from the cockpit).
Identify the descending blade side for each engine.
Remember: P-factor shifts thrust toward the descending blade side at high power/high AOA.
The engine whose effective thrust line ends up farther from the centerline (bigger moment arm) is the one that creates the worst yaw when it’s the only one running.
The engine that would leave you relying on that “worst yaw” engine is the critical engine.

For the most common conventional setup (both props clockwise from cockpit):

Descending blade is on the right side of each prop disk
Thrust shifts right
Right engine thrust becomes more outboard (bigger arm)
Therefore losing the left engine is worse → left engine is critical

Quick Reality Check: Why You Still Care (Even If You Fly a Counter-Rotator)

Even if your airplane is counter-rotating and the critical engine is effectively neutralized, PAST still matters because it explains:

why Vmc exists and why it’s not “just a number”
why engine-out handling is a controllability problem first, performance problem second
why you manage yaw/roll aggressively and clean up drag immediately

Critical engine theory isn’t about winning a trivia contest—it’s about understanding which failure produces the ugliest asymmetry and why.

Common Mistakes When Teaching or Answering This in an Oral

“Left is always critical. ”Not if it’s counter-rotating, and not in every design. Start with prop rotation.
Only talking about P-factor.P-factor is the star, but PAST is the whole cast. Examiners like seeing you understand the broader asymmetry picture.
Skipping the definition. Always start: “Critical engine is the one whose failure is most adverse to controllability and performance.”

Bottom Line

Use PAST to build the story:

P-factor usually drives the biggest yaw/moment-arm difference
Accelerated slipstream, Spiraling slipstream, and Torque pile on to make one engine-out case worse
In most conventional twins with both props rotating the same direction, that leads to a predictable result: the left engine is often critical
In counter-rotating twins, the “critical engine” concept may be minimized—but the asymmetric effects are still real, and understanding them makes you safer

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