Originally Posted by Wing Commander Fowler
Oh Joy - so all the data used to determine whether it's safe to land is cr*p then?
Green granite, Wing Commander Fowler and others
The rest of the (1970) document reads:
5. Other Prediction Methods
As a result of the failure to correlate with the system described above, two other methods were investigated. It had been suggested that the ratio of the wet to dry stopping distance of a locked wheel vehicle and of an aircraft might be the same on the same surface when the former was using plain tyres. As the Mu-Meter was a trailer it could not measure its stopping distances, but it was possible to obtain a relationship curve between the value it read on a number of surfaces at 64 k.p.h. (40 m.p.h.) and the calculated stopping distance from 113 k.p.h. (70 m.p.h.) as defined by its speed friction curve. The wet over dry ratio was then only a matter of reading off the appropriate values from the curve.
Results from Wallops Station on the Convair 990 and F4 indicated this method had considerable promise and in fact a 1:1 relationship between Mu-Meter and aircraft was achieved on a number of the surfaces so it was decided to continue this verification by further trials in the United Kingdom with a Hunter and Beverly aircraft. It soon became apparent however that these two aircraft gave different values for the wet to dry stopping distance ratio on the same surface, in fact it varied from 1 to 1.7 and therefore a system using wet to dry ratios for the prediction of stopping distance could be in error by as much as 70%, and as this was considered unacceptable the method was discarded.
Having discarded the wet to dry ratio method it was decided to determine the aircraft stopping distances on a number of surfaces with as wide a friction range as possible and to plot these against the values given by the Mu-Meter at 64k.p.h (40 m.p.h.) obtained as close to the time of the aircraft friction run as possible. In fact a technique was developed ehere the friction meter made a run immediately before and after the aircraft and the value used was a mean of the two. This method was successful and can produce individual curves for each aircraft provided sufficient points are obtained. The possibility of predicting aquaplaning for individual aircraft types and undercarriage configurations is under investigation. By making use of snow and ice covered runways it was possible to determine the Hunter correlation curve down to very low values under natural conditions. (attached figure).
6. Discussion
With the advantage of hinsight it is perhaps not surprising that the friction values of aircraft and vehicular devices do not agree. An aircraft tyre operates at far higher pressures and loads than vehicles; the aircraft uses an automatic braking system which in itself has varying degrees of efficiency depending on its design and there are differences in wheel layout, but it is perhaps surprising that the values do not agree even when a vehicle and aircraft tyre wheel brake have identical physical parameters, except for suspension.
Bearing in mind there are a number of different ways in which friction can be measured e.g. slipping wheel, locked wheel, yawed wheel with different vertical loads, tyre pressures and test speeds it is again not surprising that each equipment can give different values under the same test conditions. These are then physical facts which must be accepted, the problem is what to do about it and therefore appears to be only one sensible solution, and that is that everybody should use the same equipment.
It is suggested that a runway friction meter should be portable, rugged, accurate, able to operate at low temperatures, relatively cheap and incapable of being influenced by the operator. It is also suggested that it should be capable of providing a pictorial representation of the friction along the complete length of the runway not only for reasons given in the note to paragraph 2.8.6 in part II of Annex 14, but also to increase its usefulness by showing the extent and variation in friction caused by rubber deposits, runway damage due to jet blast etc, In addition the same equipment could be used to classify runways in their friction order if necessary by an outside agency and if airfield operators have their own equipment, they could make their own periodic check.
7. Conclusions
Correlation between aircraft and vehicular friction values at the same speed is improbable and if it does occur then it is fortuitous.
It is unlikely there is any correlation between friction coefficient values produced by equipments using different and sometime the same principles.
The only way of establishing a standard system for providing information on the presence of contaminants on runway surfaces and their effect on braking action is for all airport operators to use the same design of equipment.
References:R.W. Sugg. Tests to Determine the Usefulness of Vehicular Instrumentation to Predict Aircraft Stopping Distances S and T Memo 9/64. Ministry of Technology.
Road Research Laboratory. Tests with Heavy Load Skidding Machine to Determine Braking Force Coefficients between an aircrafdt tyre and various wet surfaces. S and T memo 8/63 and 10/64. Ministry of Technology.