Introduction
Altitude deviations may result in substantial loss of vertical separation and/or horizontal separation, which could cause a midair collision.
Traffic avoidance maneuvers, if required, usually result in injuries to passengers and crewmembers (particularly to cabin crewmembers).
This Briefing Note provides an overview of the factors involved in altitude deviations.
This document can be used for stand-alone reading or as the basis for the development of an airline’s altitude awareness program.
Statistical Data
An analysis by the U.S. FAA and by US Airways indicates that:
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Approximately 70 % of altitude deviations are the result of a breakdown in the pilot/controller communication loop; and,

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Nearly 40 % of altitude deviation events affect the critical pair constituted by FL 100 / FL 110 (or 10 000 ft / 11 000 ft).

Defining an Altitude Deviation
An altitude deviation is defined by regulations as a deviation from the assigned altitude (or flight level) equal to or greater than 300 ft.
Factors Involved in Altitude Deviations
Altitude deviations usually result from one of the following causes:
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Misunderstanding the assigned altitude;

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Use of an incorrect altimeter setting;

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Failure to level-off at the assigned altitude; or,

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Failure to reach or maintain the assigned altitude (or altitude restriction) at the point or time assigned by ATC.

Altitude deviations always are the result of a breakdown in either:

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the pilot / system interface :

.  altimeter setting, use of autopilot, monitoring of instruments and displays; or, 

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the pilot / controller interface : . communication loop.

Altitude deviations occur as the result of one or a combination of the following conditions:

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The controller assigns an incorrect altitude, or reassigns a FL after the aircraft has been cleared to an altitude;

Altitude Deviations
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.  Pilot/controller communication breakdown (mainly readback / hearback errors), e.g.:
.  Controller transmits an incorrect altitude, the pilot does not readback and the controller does not challenge the absence of readback;
.  Pilot understands and readback an incorrect altitude but controller does not hear back and does not correct the crew readback; or,
.  Pilot accepts an altitude clearance intended for another aircraft (confusion of callsigns);

.  Pilot understands and reads back the correct altitude or FL, but select an incorrect altitude or FL , e.g. because of :
.  confusion of numbers with an other element of the message (e.g., speed, heading or flight number);
.  expectation / anticipation of another altitude or FL;
.  interruption / distraction; or,
.  breakdown in crew crosscheck and backup;

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