Frequently Asked Questions




What are the differences between CFMEs and Decelerometers?

A decelerometer (DFD) is used to measure a short section of the runway; this is sometimes called a spot check. The decelerometer is a small device that is mounted inside a suitable ops vehicle. The vehicle is brought up to 20mph and then the brakes are applied resulting in the vehicle inducing a full locked wheel skid. The decelerometer then measures the peak horizontal g force during the skid. In low friction surface conditions, typical values would be less than 40% g.

A CFME (DFT) is a completely different device that has been specifically designed to measure PEAK friction from a tire which is partially locked (between 12 to 20%); The forces on the tire are measured. The horizontal force is then divided by the vertical force which gives the MU value (which has no units). This type of system is capable of continuously measuring values in both winter contaminate and maintenance (summer) testing.

Specific attention to CFME systems that only measure a single force will not compensate for changes in loading. The DFT contaminate measurement wheel, although not approved for use by the FAA, does measure contaminate depths and is valuable data during winter operations.

No. A decelerometer can only be used during contaminated surface conditions (winter). If you use a decelerometer on a dry surface, the results you will generate will only show the efficiency of the vehicle’s braking system and not the tire-to-surface friction.

Good macrotexture means that the surface has channels (or grooves) that water can be easily pushed in to so the tire can remain in contact with the surface.

For good low-speed skidding resistance, a harsh micro-texture is essential to provide friction between an aircraft tire and a wet runway surface. It assists in penetrating the last thin film of water and provides effective contact between runway and tire. However, a very harsh micro-texture can cause increased tire wear without a proportional increase in wet friction.

Poor macrotexture means that the surface is smooth, and above slow speeds, the tire easily rides up onto the water and aquaplanes.


What is Hydroplaning / Aquaplaning?

This is when the tire is not in contact with the pavement surface, resulting in zero frictional properties and zero steering ability. The microtexture of the surface can not break through the water film and/or the macrotexture and tire cannot displace enough water. This condition is usually noticeable by the shiny surface it leaves.


Braking Action Definitions

Good: Braking deceleration is normal for the wheel braking effort applied. Directional control is normal. Mu value of 40 & above is assigned

Good to medium: Mu range of 39 – 36

Medium (Fair): Braking deceleration is noticeably reduced for the wheel braking effort applied. Directional control may be slightly reduced. Mu range of 35 – 30

Medium to Poor: Mu range of 29 – 26

Poor: Braking deceleration is significantly reduced for the wheel braking effort applied. Potential for hydroplaning exists. Directional control may be significantly reduced. Mu range of 25 – 21

Nil: Braking deceleration is minimal to non-existent for the wheel braking effort applied. Directional control may be uncertain. Note: Taxi, takeoff, and landing operations in Nil conditions are prohibited. Mu value of 20 & below is assigned