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In other words, the aquaplaning speed is a threshold from which friction forces diminish severely.
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C3.4 Correlation between reported μ and braking performance
C3.4.1 Information provided by the airport authorities
Airport authorities give measurements of a runway friction coefficient. The results are published via a standard form defined by the 1981 ICAO AGA meeting called SNOWTAM (See Figure C11).
Figure C11
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A SNOWTAM contains:
.
The type of contaminant,
.
Mean depth for each third of total runway length,
.
Estimated braking action,
.
Reported μ.
Some operators have requested that Airbus provide contaminated runway performance data with Reported μ or Estimated Braking Action as an entry, in lieu of the type and depth of contaminant.
REPORTED MU ESTIMATED BRAKING ACTION
0.4 and above GOOD - 5
0.39 to 0.36 MEDIUM/GOOD - 4
0.35 to 0.30 MEDIUM - 3
0.29 to 0.26 MEDIUM/POOR - 2
0.25 and below POOR - 1
9 - unreliable UNRELIABLE - 9
C3.4.2 Difficulties in assessing the effective μ
The two major problems introduced by the airport authorities evaluation of the runway characteristics are:
.
The correlation between test devices, even though some correlation charts have been established.
.
The correlation between measurements made with test devices or friction measuring vehicles and aircraft performance.
These measurements are made with a great variety of measuring vehicles, such as:
Skidometer, Saab Friction Tester (SFT), MU-Meter, James Brake Decelerometer (JDB),
Tapley meter, Diagonal Braked Vehicle (DBV).
Refer to ICAO, Airport Services Manual, Part 2 for further information on these measuring
vehicles.
The main difficulty in assessing the braking action on a contaminated runway is that it
does not depend solely on runway surface adherence characteristics.
What must be found is the resulting loss of friction due to the interaction tire/runway.
Moreover, the resulting friction forces depend on the load, i.e. the aircraft weight, tire
wear, tire pressure and anti-skid system efficiency.
In other words, to get a good assessment of the braking action of an A340 landing at
150,000 kg, 140 kt with tire pressure 240 PSI, the airport should use a similar spare
A340... Quite difficult and pretty costly!
The only way out is to use some smaller vehicles. These vehicles operate at much lower
speeds and weights than an aircraft. Then comes the problem of correlating the figures
obtained from these measuring vehicles and the actual braking performance of an
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