. ICAO : FL one zero zero / FL one one zero;
.  U.K. NATS: FL one hundred / FL one one zero;

.  Mindset leaning to focus only on “one zero” and thus to more easily understand “10 000 feet”;
                                                                                       Getting to Grips with                                            Approach-and-Landing Accidents Reduction

.  
Failing to question the unusual (e.g. bias of expectation or routine on a familiar SID or STAR) and/or,

.  
Interpreting subconsciously a request to slow down to 250-kt as a clearance to descend to FL 100.

Transition Altitude / Level
As indicated in Briefing Note 3.1 - Altimeter Setting – Use of radio Altimeter, the transition altitude / flight level can be either:
.  
Fixed for the whole country ( e.g. FL 180 in the United States);

.  
Fixed for a given airport (as indicated in the approach chart); or,

.  
Variable as a function of QNH (as indicated in the ATIS message).

Depending on the airline’s / flight crew’s usual area of operation, changing from fixed transition altitudes/FL to variable transition altitudes/FL may result in a premature or late change of the altimeter setting.
An altitude constraint (expressed in terms of FL in climb or expressed in terms of altitude in descent) may advance or delay the change of altimeter setting and cause crew confusion.
In countries operating with reference to the QFE, when below the transition altitude or FL, the readback should indicate the altimeter reference (i.e., QFE).
Altitude Deviations in Holding Patterns
In holding patterns controllers rely on pilots maintained the assigned altitude or to descend to the new cleared altitude.
The overlay of aircraft tags on the controller’s radar display does not allow the immediate detection of an impending traffic conflict.
Controllers, therefore, assume that a correctly readback clearance will be correctly complied with.

Altitude Deviations
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Secondary surveillance radar’s (SSR) provide conflict alerts but no resolution advisory; accurate and clear pilot / controller communications are essential when descending in a holding pattern.
Two separate holding patterns may be controlled by the same controller, on the same frequency.
　
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