Actual height 1000 ft AFL
1000 ft
Figure 1

In Figure 2, the actual QNH is an usually high 30.XX in.Hg but the altimeter setting was mistakenly set to a more usual 29.XX in.Hg, resulting in the actual altitude / height being 1000 ft higher than indicated.

QNH 30.XX in.Hg Altimeter error 1000 ft
Figure 2

Confusion between altimeter setting units (i.e. hPa versus in.Hg) leads to similar errors in the actual altitude and actual height above airfield elevation.
In Figure 3, an actual QNH of 991 hPa was mistakenly set on the altimeter as 29.91 in.Hg (equivalent to 1012 hPa ), resulting in the actual altitude / height being 640 ft lower than indicated.

Actual height 1360 ft AFL
640 ft
Figure 3

Setting the altimeter reference
In order to eliminate or lessen the risk associated with the use of different altimeter-setting units or with the use of unusual (low or high) altimeter-setting values, the following rules should be used by controllers (when recording the ATIS message or when transmitting the altimeter-setting) and by pilots (when reading back the altimeter-setting):
.  All digits as well as the unit (e.g., inches or hectopascals) should be indicated.
A transmission such as “altimeter setting six seven” can be interpreted as 28.67, 29.67 or
30.67 in.Hg, or as 967 hPa.
Indicating the altimeter-setting unit prevents confusion or allows detection and correction of a previous error.

.  When using inches of mercury (in.Hg), “low” should precede an altimeter setting of 28.XX in.Hg and “high” should precede an altimeter setting of 30.XX in.Hg.
The U.S. FAA accepts this practice, if deemed desirable by regional or local air traffic services.

Altimeter Setting – Use of Radio Altimeter
Page 2

AIRBUS INDUSTRIE
    Flight Operations Support

The incorrect setting of the altimeter reference often is the result of one or more of the following factors:
.  
High workload;

.  
Deviation from normal task sharing;

.  
Interruptions and distractions; and,

.  
Absence of effective cross-check and backup between crewmembers.

Adherence to the defined task sharing (for normal or abnormal / emergency conditions) and the use of normal checklists are the most effective lines-of-defense against altimeter setting errors.
Use of Metric Altimeter
Using metric altitudes in certain countries (such as the Commonwealth of Independent States [CIS] and The People’s Republic of China) also requires adapted procedures for setting the selected altitude on the FCU and the use of metric altimeters (or conversion tables) for reading the altitude in meters.
Reset of Altimeter Setting in Climb or Descent
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, depending on 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 setting of the altimeter reference.
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 the altimeter reference and cause crew confusion.
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