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 at 40 mph and 60 mph the full length of the runway averaging every 250 feet of surface measurement
Can Decelerometers be used for Maintenance testing as per AC 150/5320-12C?
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-runway friction.
When does runway friction deterioration occur?
Over time skid-resistance of runway pavement deteriorates due to jet aircraft rolling or braking on the pavement and the accumulation of contaminants, chiefly rubber, on the pavement surface. The effect of these two factors is directly dependent on the volume and type of aircraft traffic.
Contaminants include rubber deposits, water (rainfall), snow, ice, and slush -- all cause friction loss on runway surfaces. Removal and runway treatment for snow, ice, and slush are covered in AC 150/5200-30C. The most persistent contaminant problem is deposit of rubber from tires of landing jet aircraft. Rubber deposits occur at the touchdown areas on runways and can be quite extensive. Heavy rubber deposits can completely cover the pavement surface texture causing loss of aircraft braking capability and directional control, particularly when runways are wet .
What is the difference between Macro and Micro texture?
Good macrotexture means that the surface has channels (or grooves) that water can be easily channeled so the tire can remain in contact with the surface.
Microtexture is the course aggregate material that makes direct contact with the tire. 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.
What is Hydroplaning / Aquaplaning?
Hydroplaning is the loss of contact of the tire to the pavement, 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.