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I'm studying the principles of flight, and I'm having some unanswered questions in my mind regarding the turn and slip indicator.

I'm reading that a turn and slip indicator displays the slip or skid of the turn. It probably has something to do with directional stability.

But the questions I'm having are:

  • Why does it matter if the airplane is in a skidding or slipping turn?

  • What are the consequences of skidding or slipping turn?

  • Is it dangerous to be in a skid or slip?

  • Why would a pilot would like and/or not like to be in a skid or slip?

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The turn and slip instruments are combined for convenience and by convention more than anything else, slip is important whether you are in a turn or not. When you slip it means one wing is more directly pointed into the airflow, which increases lift for that wing in comparison to the other. At a high angle of attack, for instance a climb and close to stall speed, a slip can put one wing into a stall, causing the non-stalled wing to flip the airplane. When done deliberately it's an aerobatic maneuver called a snap roll, when it's not deliberate it's a major cause of stall-spin accidents on take-off and approach. So slip is dangerous in some situations.

In level flight slip creates drag, so it's worth keeping the ball in the middle when safety is not a concern as well.

As for deliberate slip as I said before slip can be induced for aerobatic maneuvers like snap rolls and spins, slip can also be used for non-aerobatic maneuvers like side-slips, where you put the airplane out of balance to increase drag, allowing more rapid descents without increasing airspeed.

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    $\begingroup$ I would like to add this, it's also one answer for the question: Why does a Turn and slip indicator matter?. My book describes that the turn and slip indicator works as a back-up in case of a malfunctioning artificial horizon / attitude indicator. Is this true in practise? $\endgroup$ – Julian Nov 27 '19 at 10:41
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    $\begingroup$ That's a separate question @Julian, why not ask it? $\endgroup$ – GdD Nov 27 '19 at 10:59
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    $\begingroup$ I did so, please have a look: aviation.stackexchange.com/questions/72047/… $\endgroup$ – Julian Nov 27 '19 at 13:46
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The "slip" part of the Turn and Slip Indicator (T/S) measures the lateral (transverse) acceleration of the airplane. When the slip indicator is centered, the pilot and passengers will feel gravity directly in-line with the seats; otherwise, they will feel a lateral sway, which doesn't provide the best ride experience, especially if martini spill is involved. Obviously, this is important whether you are in a turn or simply flying level.

In symmetric flight with multiple engines providing the same thrust, a zero lateral acceleration is very close to having zero aerodynamic sideslip. Achieving flight with minimal sideslip improves the fuel economy since sideslip introduces unnecessary drag for symmetric flight. In practice, however, non-zero sideslip may develop because of minor aerodynamic asymmetry, prop wash, and especially if you have one engine inoperative (even with T/S centered).

Having non-zero slip indication in itself is not dangerous. All Part 23 and 25 aircraft certify by performing steady-heading sideslip up to maximum rudder pedal or the limit of their lateral control. When you decrab in a crosswind, you will intentionally generate sideslip. However, an excessive slip coupled with high angle of attack is dangerous as it can lead to potentially irrecoverable spin-stall.

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In a small aircraft, the turn indicator (and the slightly better turn coordinator) is usually electrically powered, while the attitude indicator is vacuum powered. If the attitude indicator fails, either mechanically or do to vacuum fail, the turn indicator can assist with keeping the wings level in instrument conditions.

The slip indicator is merely a ball in a fluid and is not subject to any electric or vacuum failure, and can be a backup for an electronic flight display's slip-skid indicator. In attempting a coordinated turn, the ball can be "stepped on" via the rudder to prevent undesirable yaw. In some aircraft, rudder isn't necessary in a turn or initial bank as yaw is negligible.

Under Instrument Meteorological Conditions (IMC), a skid at or near stall speed indicates a spin (usually undesirable), and the instrument can assist with recovery. A slip at speeds well above stall with rapidly increasing airspeed indicates a spiral. The spiral may be due to misleading attitude indicator, and the indicator and ball can assist in recovery from a spiral.

In a multi-engine aircraft under instrument conditions, yaw (and heading change) is one of the first indications of an engine failure of some sort due to asymmetric thrust, and "stepping on the ball" and raising the wing of the dead engine can restore stable flight.

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