What's the meaning of “straight and level”?

Question

Indeed a very strange question.

My understanding of straight and level is: The aircraft is flying at a constant heading and constant altitude, and maybe at a constant speed, and this has nothing to do with the aircraft attitude. However I was reading this in "Big Red Today":

"If you just keep it straight and level, the chances of surviving are better," Ms. [...] said. "But as soon as you start to turn, you put yourself at risk of a stall-spin.

Here this is clearly about maintaining the aircraft neutral attitude while landing, so losing altitude, and probably speed.

Is this use of "straight and level" correct? What is the definition of straight and level, if any, regarding heading, attitude, speed, altitude?

^{Note: Please explain for all parameters mentioned above to remove all ambiguities.}

Answer 1

Is this use of "straight and level" correct? What is the definition of straight and level, if any, regarding heading, attitude, speed, altitude?

In the airplane flying handbook the FAA defines straight and level flight as

Straight-and-level flight is flight in which heading and altitude are constantly maintained.

My understanding and the instruction I have received has always lead me to understand that Straight-and-Level is independent of Attitude and Speed. However to maintain straight and level flight at a given speed you will have to put the aircraft in a given attitude. Generally speaking at lower speeds aircrafts have a higher nose up attitude to maintain level flight and a more nose down attitude at high speeds to maintain level flight but this can vary by design and wing mounting.

In this case I would say it was used a bit improperly. The statement should have been more along the lines of "the pilot should have maintained his heading and conducted a straight ahead emergency landing. You can read more on why that is here.

You can refer to the full NTSB report of the accident here for all the details.

Answer 2

I'll quote from Rod Machado's first ground school lesson:

Straight flight means the airplane's nose remains pointed in one direction and the wings are parallel to the earth's horizon. Level flight means the airplane doesn't gain or lose altitude.

_{(my emphasis)}

Therefore, there are three constraints for "straight and level" flight:

1. Constant heading (i.e. yawing the plane while keeping wings level doesn't count)
2. Wings level
3. Constant altitude (i.e. vertical speed indicator reads zero)

It does not constrain:

1. Speed
2. Pitch

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I believe the author of the article unintentionally used the term wrong. I bet what he meant is "straight and [wings] level", not "level" in "level flight", which means constant altitude.

The point in the article is that one should not be making turns while in close proximity with the ground, as keeping the wings level will provide you maximum lift against gravity, thereby reducing your chances of hitting the ground, or even if it does, reduce your impact energy by lowering the velocity change in the vertical direction, or let the landing gear absorb the impact before it hits the fuselage.

Answer 3

Straight and level is a kind of the broader steady flight. Steady flight is "a special case in flight dynamics where the aircraft's linear and angular velocity are constant in a body-fixed reference frame."

As constant velocity means no acceleration, straight and level is maintaining altitude, heading, and speed.

A plane on approach would qualify as a steady longitudinal descent.

The omaha.com article's use of straight and level is technically incorrect, but colloquially can be forgiven.

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The Wikipedia article is largely based on McClamroch, N. Harris (2011). Steady Aircraft Flight and Performance. Princeton, NJ: Princeton University Press. ISBN 9780691147192.