Seeing the Munich Airport has just taken delivery of some 9-5 BioPower emergency vehicles to compliment their Saab friction testing vehicles, I thought it might be high time to mention a little about these fiction testers. Saab are pretty well known as pioneers in this field and Saab vehicles are still fitted out today as friction testing vehicles for use all around the world.
Saab Friction tester
Friction testing was initially done with a trailer system, called a skiddometer. I’m not kidding.
As the administrations of busy airports found that trailers had certain disadvantages SAAB started in the late sixties to develop a friction-measuring unit, the SAAB Friction Tester, SFT. A fifth wheel, the friction measuring wheel, was installed in the rear of a SAAB car model 99.
Whilst the earliest Saab friction testers were 99s, the principal has been applied to all Saab models since, with 900s, 9000s and 9-5s all getting the conversion at one time or another.
Why measure friction?
Flight Safety is the main reason for measuring friction. As the transport aeroplanes became larger it became also more important to check friction in a better way than making skid tests as mentioned above. Scandinavia, particularly Sweden, has taken a considerable part in the development of friction measuring technique.
Among reasons for friction measurements are:
* Determine friction characteristics of runways under winter conditions
* Verify friction characteristics of new or resurfaced runways
* Assess periodically the slipperiness of paved runways when wet
* Assess the effect on friction when drainage characteristics are poor
* Assess friction of runways becoming slippery under unusual conditions
How friction testing works
The measuring wheel was connected to the rear wheels of the car via chains and sprocket wheels. This means that the skiddometer principle is used and some 80 to 85 per cent of the braking force is used as propelling force. By selecting the teeth on the sprocket wheels and the diameter of the measuring wheel suitably the desired slip could be obtained. This slip is selected for operational measurements in order to reduce tire wear.
The friction measurements come from a fifth wheel accessed through the hatchback and connected to the rear wheels. The fifth wheel is lowered down and forced onto the pavement with a down force of 300lbs. A built-in 10 to 15 percent slip helps continuously calculate the friction of the surface. The friction tester is used to determine the braking conditions aircraft should expect on a particular runway during winter snow and ice conditions. Arriving and departing aircraft use the numbers to determine the safety of the runway as well as the necessary braking distances required for their particular operation.
Modern friction testing Saabs are primarily customised by The Scandinavian Airport and Road Systems AB (SARSYS).
The SFT is programmed to measure in accordance with regulations issued by authorities such as ICAO, SCAA and the FAA and is designated for both operational and maintenance testing.
* High-performance front-wheel drive car
* Excellent maneuverability
* First class working environment for the operator
* Proven reliability in all climates, from the coldest parts of Northern Europe and Canada to very hot places like Saudi Arabia and Singapore
* Large space in the rear provides unimpeded access to the measuring system, making service and maintenance work easy and comfortable.
* Entire measuring system contained within the car, which retains it’s exceptional driving qualities.
* Self-contained configuration provides speed and smoothness of operation.
In operational situations, the vehicle is immediately available for a measuring run. A runway friction report is available within just a few minutes after the start of the run. The SFT can be equipped with a TRACR II® system or radio data link to transmit friction data directly to a PC.
For maintenance measuring, which stipulates a wet runway, the SARSYS SFT is available with a watering system. The water tank has a volume sufficient for 23,000 feet (7,000 meters) of runway with a 1mm water layer.