Flight Operations Briefing Notes Supplementary Techniques Altimeter Setting - Use of Radio Altimeter

 
Flight Operations Briefing Notes
 Supplementary Techniques
Altimeter Setting - Use of Radio Altimeter
Introduction

Operators with international routes are exposed to different standards in terms of:
Altitude measurement, using different units (i.e., feet or meters);

Altitude reference setting (i.e., baro setting), using different units (i.e., hectoPascal or inch-of-mercury);

Altitude reference for departure and approach, using QNH or QFE; and,

Environmental conditions (i.e., rapid atmospheric pressure changes and/or low OAT operation).

This Flight Operations Briefing Note provides a review and discussion of  the following aspects, highlighting the lessons learned from incidents / accidents (particularly during approach-and-landing) :
Barometric-altimeter reference (QNH or QFE);

Use of different units for altitude measurement and reading (i.e., feet versus meters) and altimeter setting (i.e., In.Hg versus hPa);

Setting of baro-altimeter bugs (as applicable) and radio-altimeter DH;

Radio-altimeter callouts; and,

Low-OAT operation.

Statistical Data

Deviations from the intended vertical flight profile, caused by omission of an action or by an incorrect action (including errors in setting the altimeter reference),  are frequently observed during line operation.
The lack of situational awareness, particularly the lack of vertical situational awareness, is a causal factor in 50 % of approach-and-landing accidents (this includes most accidents involving a CFIT) (Source: FSF Flight Safety Digest Volume 17 & 18 – November 1998 / February 1999).
QNH or QFE ?

Some operators set the altimeter to QFE, for takeoff and approach-and-landing, in areas of operation where the ATC and the majority of other operators use QNH.
This requires adequate SOPs for altimeter-setting and for conversion of assigned altitudes into heights.
The difference between the QNH and QFE is indicated in approach area chart, e.g. :
LFBO ( Toulouse Blagnac ) :

“ ELEV 499 ft / 152 m (18 hPa) ” :

QNH 1014 hPa            QFE = 996 hPa,

3000 ft QNH = 2500 ft QFE.

Pilots should be also aware of possible exceptions, such as airports operating with  “QFE only” in a country where QNH is used (such exceptions are indicated on  the applicable approach chart).
Aircraft fitted with electronic flight instrument systems (EFIS) may be capable of using either QNH / QFE or QNH-only.
Altimeter-setting Units

Operators with international routes are exposed to the use of different altimeter setting units:
Hectopascals (hPa), previously referred to as milibars (mb);

Inches-of-mercury (in. Hg); or,

Milimeters-of-mercury (mm.Hg), on earlier eastern-built aircraft.
　
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