BOAC, et al. Take care not confuse the runway maintenance term ‘slippery when wet’ with the non-standard use of ‘slippery’ when discussing landing performance, particularly in Boeing references where slippery often replaces ‘icy’ or ‘contaminated’.
A runway may be described as ‘Slippery when Wet’ when the runway friction is less than the desired value from a construction / maintenance viewpoint, see UK CAA FODCOM 2003_23 and UK CAA CAP 683 ‘The Assessment of Runway Surface Friction’. These friction measurements are not correlated directly to any specific aircraft type but are intended as guidelines for aerodrome operators on areas of the runway that require maintenance. … any performance calculations or adjustments resulting from the dissemination of this information is the responsibility of the aircraft operator and the commander, and not the aerodrome operator. … UK Air Pilot AD 1.1.1 section 15 Wheel Braking Action on Wet and contaminated Runways.Other references to the runway maintenance aspect include Transport Canada TP 312.
The measured (operational) value of friction (braking action) used to determine landing performance should be used with care. See the presentation Managing the Threats and Errors during Approach and Landing. (www.flightsafety.org/ppt/managing_threat.ppt). Note in particular the phrase on slide 27 - there is no overall accepted certification / operational correlation between mu meters and airplanes. Moreover the mu readings themselves are unreliable in wet snow and slush.
One of many hazards of terminology is that Boeing misuse ‘slippery’ (not an ICAO term). A Boeing view of (their) landing problems is given in presentations Stopping on Slippery Runways (www.pilots.or.kr/upfile/aip/9AAAA_StoppingonSlipperyRwys2.ppt) and Boeing slippery runways (http://uk.geocities.com/
[email protected]/alf5071h.htm), slide 15 onwards deals with the different descriptors. Note the differences between ‘Runway mu (measured) and Airplane mu (calculated).
The presentation notes for slide 17 read – “For Method 1, Boeing assigns a scale to the descriptor terms used in the QRH in order to make the landing distance calculations. This scale is airplane braking coefficient, or Mu-Airplane, with a range from 0 to 0.4.
Mu-Airplane is a calculated value of how the airplane systems convert the available runway friction into effective stopping performance. This calculation takes into account airplane GW, lift, CG and antiskid system performance.
Method 3 is the runway friction as measured by an airport ground vehicle. A scale called runway friction coefficient, or Mu-Runway, is used with a range from 0 to 1. This is the measured value of the point at which the tire will skid on the runway, and does not account for any airplane parameters (GW, CG, etc. ).
Thus, Method 1 is a calculated airplane capability and Method 3 is the measured tire skid point. Flight crews should be aware that these are two entirely different things and these scales cannot be directly compared.”
The presentation also identifies a hazard of autobrake on low friction surfaces.
For additional reference see:
NPA_14_2004 ‘Operations on Contaminated Runways’ via EASA web site.
UK CAA AIC 15(P92)/2006 ‘Risks and factors associated with operations on runways affected by snow, slush or water.’
Previous threads on ‘contaminated surfaces’, and ‘runway braking action terminology’.
:rolleyes: