Definition

A term used by pilots to characterize the deceleration associated with the wheel braking effort and directional controllability of the aircraft.

Description

While there has been no official ICAO definition, the term "braking action" has been widely used in the aviation industry in various contexts. On the one hand, it defines the stopping capability of an aircraft using wheel brakes and is related to pilot reports. It can also be used to describe the estimated surface friction on the ground measured by a friction measurement device and reported as aircraft stopping capability. The SNOWTAM format uses the term "estimated surface friction" which is an assessment of the slipperiness of the surface by the ground staff based upon all information available. It should be noted though, that "braking action" and "surface friction" are not to be used interchangeably and that the latter is merely used as a proxy to provide and idea of the former.

The relation between "braking action" and "surface friction" is not straightforward. It depends on a number of factors, such as the fiction measuring equipment, the surface conditions and the person conducting the measurements. The table below with associated descriptive terms was developed from friction data collected only in compacted snow and ice and should not therefore be taken to be absolute values applicable in all conditions.

     
It should be noted however, that if the surface is affected by snow or ice and the estimated surface friction is reported as "good", pilots should not expect to find conditions as good as on a clean dry nmway (where the available fiiction may well be greater than that needed in any case). The value "good" is a comparative value and is intended to mean that aeroplanes should not experience directional control or braking difficulties, especially when landing. The figures in the "Measured Coefficient" column are given as an indication. At each aerodrome a specific table can be developed according to the measuring device used on the aerodrome and according to criteria set or agreed by the State. The PI values given will be specific to each friction measuring device as well as to the surface being measured and the speed employed.

Generally, readings from a friction measuring device should be used only as part of an overall runnway condition assessment. A major difference between the decelerometer type of devices and the other types is that when using the decelerometer type the operator is an integrated part of the measuring process. In addition to canying out the measurement, the operator can feel the behaviour of the vehicle where the decelerometer is installed and by that feel the deceleration process. This gives additional information in the total assessment process.

It has been found necessary to provide assessed surface condition information, including estimated surface friction, for each third of a nmway. The thirds are called A, B and C. For the purpose of reporting infonnation to aeronautical service units, section A is always the section associated with the lower runway designation number. Wien giving landing information to a pilot before landing, the sections are however referred to as first, second or third part of the runway. The first part always means the first third of the runway as seen in the direction of landing (i.e. the touchdown zone).

The following standard phraseology is used when providing pilots with information about the runway braking action or surface friction:

• ESTIMATED SURFACE FRICTION (descriptive term). Example: ESTIMATED SURFACE FRICTION MEDIUM TO GOOD
• BRAKING ACTION REPORTED BY (aircraft type) AT (time) (descriptive term). Example: BRAKING ACTION REPORTED BY A320 AT 1000 MEDIUM

It should be noted the standard phraseology stresses on the surface friction being an estimate and the braking action being a pilot report.
 

Further Reading

• ICAO Annex 14: Aerodromes
• ICAO Doc 9137: Airport Services Manual, Part 2: Pavement Surface Conditions