Specially treated runways

[PEI_3721]

Jw,
What next, you might dare to ask the authority what is appropriate – either operators/manufacturer’s judgement or don’t do it at all.

Checking CS 25 AMC 25.1591(~page 665) ‘The derivation and methodology of performance information for use when taking-off and landing with contaminated runway surface conditions’, which is primarily guidance for manufacturers, but has information relevant to operations.

Some of the important points are: “Due to the wide variation in possible conditions when operating on contaminated runways and the limitations inherent in representing the effects of these conditions analytically, it is not possible to produce performance data that will precisely correlate with each specific operation on a contaminated surface. Instead, the performance data should be determined for a standardised set of conditions that will generally and conservatively represent the variety of contaminated runway conditions occurring in service”.
Hence good risk mitigation by downgrading the reported braking action.

Re sand (definitons): “Specially Prepared Winter Runway.
A runway, with a dry frozen surface of compacted snow and/or ice which has been treated with sand or grit or has been mechanically or chemically treated to improve runway friction. The runway friction is measured and reported on a regular basis in accordance with national procedures.”
N.B. this conflicts with TALPAs means of determining braking action vs ‘friction’
… “The responsibility for relating this data to a friction index measured by a ground friction device will fall on the operator and the operating authority.”
There is also a note on the default values of friction (assumptions used in perf calcs) “Note: For a specially prepared winter runway surface no default friction value can be given due to the diversity of conditions that will apply.”

And the final ‘catch all’ which appears to be the basis of UKCAA airfield policy; “Operation on runways contaminated with water, slush, snow, ice or other contaminants implies uncertainties with regard to runway friction and contaminant drag and therefore to the achievable performance and control of the aeroplane during take-off, since the actual conditions may not completely match the assumptions on which the performance information is based. Where possible, every effort should be made to ensure that the runway surface is cleared of any significant contamination.”

The whole section of the AMC is well worth reading, if only to enlighten management as to some of the risks in such operations.