Friction Measuring in GRF

In GRF regulations, the inspector has to consider several inputs when he/she is assessing the overall condition of the runway.

Among the five different input channels in the runway assessment, the friction measurement is the only objective measurement regarding the runway’s slipperiness. However, the measured friction values are not included in the Runway Condition Assessment Matrix (RCAM) table. The GRF states that measured friction should be used comparatively.

This post goes into the most common contaminant combinations in real life not included in the RCAM table and measured friction values as assessment criteria for runway conditions in these situations.

NON-RCAM combinations observed in real life

1. Dry snow on top of frost

As the most common NON-RCAM combination, this situation occurs when there is frost build-up on the runway, and a snowfall front or wind is blowing on top of the frost.

2. Frost on top of ice

This situation especially occurs in northern airports when there is ice or frost build-up on the runway’s edges. Thus, the frost is building up on top of the ice.

3. Slush on top of water

For example, when chemicals have been used on the runway, the chemicals make the runway surface wet. If a snowfall front comes, the snow floats on top of the chemical layer and forms slush or wet snow.

4. Frost, water and slush

the frost usually melts gradually, mainly during spring, when there is frost build-up on the runway during the night and then the sun warms the runway.  This means that there might be water on some parts of the runway, slush on some other parts and others might still have frost.

5. Wet snow on top of water

Again, when chemicals have been used on the runway, they make the runway surface wet. If a snow front comes over the airport, it floats on top of the chemicals and forms a wet snow layer on top of the water.

6. Slush on top of ice

In rare cases, slush on top of ice might also be observed in real life.

We hear that the inspectors are pressured about the upcoming Global Reporting Format (GRF) for increased responsibility. The main tool for reporting the runway conditions is the inspectors’ subjective view of the runway condition. Using a friction tester as a supporting tool, the final decision is still up to the inspector.  He can feel more comfortable choosing the runway condition code when he has an objective backup for his/her decision, especially in less clear NON-RCAM situations.

Based on our analysis of about 70,000 runway reports, there are cases where the pure RCAM selection can cause safety issues by giving a higher runway condition code than the friction readings indicate. There are also many situations where runway closure would occur based on RCAM, but at the same time, the friction tester is giving higher readings. This can generate unnecessary runway closures, which are economically unwanted to the airport. This pressures the inspector to upgrade or downgrade the RCAM value, which should be based on total assessment. The friction tester is valuable here as the inspector can look for all available information and reach a decision.

Safety and regularity in the use of measured friction

By default, the RCAM table does not include a friction column. However, measured friction values are the only objective measured value from the runway surface. Thus, they are important information, especially in contaminant combination situations of assessing the upgrade and downgrade of runway condition code. However, some basic issues should be taken care of before friction as one assessment criterion or input channel can be used:

1. Use friction tester’s specific guidance

The surface friction tester should have specific guidance.

2. Take into account the friction tester’s operational envelope

The inspector should consider the maximum contaminant depth limits per contaminant type when measuring friction according to the runway friction tester’s operational envelope. For example, if the layers of contaminant on the runway are thicker than the operational envelope specifications, they might form a wave in front of the measuring wheel. In this case, the measured friction values are not reliable.

3. Collect and analyse data

The regulations encourage the use of friction testers in a comparative way. This is why data needs to be collected (at least about one year’s data) and compared to reported RCRs. Later on, the inspector can compare the measured friction value to this data to select the runway condition code value.

4. Declare friction tester’s annual maintenance and weekly calibration check programmes

The airport operator must declare the friction tester’s annual and weekly maintenance and calibration programme or at least the calibration check. This assures that the measured values will remain repeatable on the same surface.

5. Declare inspectors’ initial training and periodic refreshment training programs

The inspectors’ initial training for the friction tester and periodical refreshment training programmes should be declared to maintain the right operation policies and methods when measuring friction.

To conclude, measured friction can safely be used as one assessment criterion for the runway condition through the above conditions. Friction values are essential information, especially when assessing the upgrade or downgrade of the runway condition code and relieving the inspector’s stress. Data collection is the key to success. The comparative is a safe way to use friction tester when GRF becomes effective.

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