The radio altimeter is an important tool to help minimise the risk of CFIT, because it provides an independent and unambiguous warning of proximity to the ground, regardless of any navigational uncertainty or error, e.g. mis-setting of the barometric altimeter sub-scale.
Standard operating procedures should include procedures for the use of the radio altimeter. Specific procedures should be mandated for each type of approach that is likely to be flown. For example the procedures used for a day VFR approach may well be different to those for a night VFR approach and those for an IFR approach. Where there is a radio altitude bugging facility, procedures for setting the warning alert should be specified along with actions to be completed by the crew if/when the warnings are initiated. Consideration should be given, where appropriate, for any differences in procedures for operations with multi-crew, two-crew and single-pilot operations.
Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.2 — Terrain recommends that, "To enhance the flight crew’s terrain awareness, the call “radio altimeter alive” should be made by the first crewmember observing the radio-altimeter activation at 2,500 feet.
"The radio-altimeter indication then should be included in the instrument scan for the remainder of the approach." the briefing note continues.
"Flight crews should call radio-altimeter indications that are below obstacle-clearance requirements during the approach. The radio altimeter indications should not be below the following minimum heights:
- 1,000 feet during arrival until past the intermediate fix, except when being radar-vectored;
- 500 feet when being radar-vectored by ATC or until past the final approach fix (FAF); and,
- 200 feet from the FAF to a point on final approach to the landing runway where the aircraft is in visual conditions and in position for a normal landing, except during Category (CAT) II instrument landing system (ILS) and CAT III ILS approaches.
The radio altimeter is not, however, an easy instrument to monitor; its indications depend on the terrain being overflown, It does not fit naturally into the instrument scan, and any procedure which depends on crew monitoring to make call outs based on the radio altimeter suffers from high error rates. There is however an alternative: ‘talking radio altimeters’ do not suffer from stress, rarely fail to make the appropriate callouts, and procedures based on them are reliable. One major European Operator incorporated the following simple table into it’s Ops Manual:
|Automatic Call Out
||MUST be level or on an approach
||MUST be on an approach, nearing DH
||MUST be in sight of Threshold (except LVPs)
||MUST NOT be still in sight of Threshold
|ELSE GO AROUND
The specific call outs are pin-programmable, but those above are typical, the table can be adjusted accordingly.
Notwithstanding the benefits of setting a minimum radio altimeter indication for all phases of an approach, it should be stressed that when approaching a cliff top airfield from over the sea – such as the easterly runway at Jersey – or an airfield in hilly terrain from over a valley, use of the radio altimeter on the approach is of limited benefit and may actually give an unsafe view of terrain clearance during the final approach phase. It should always be borne in mind that the radio altimeter in isolation has no “look forward” capability.
Flight Safety Foundation
The Flight Safety Foundation ALAR Toolkit provides useful training information and guides to best practice. Copies of the FSF ALAR Toolkit may be ordered from the Flight Safety Foundation ALAR website