Friction Testing – Measurement of Pavement Surface Friction – Airport Maintenance

Overview

Friction testing of airport runway pavement surfaces is a highly technical and essential process for aviation safety. It quantifies the interaction between aircraft tires and the pavement, which is crucial for braking capability and directional control during all ground operations—especially landings, takeoffs, and aborted takeoffs. The amount of available friction varies with weather, surface contamination, pavement wear, and maintenance activities. Regulatory agencies such as ICAO and the FAA mandate regular measurement and monitoring of runway friction to prevent skidding, hydroplaning, and loss of control.

Friction testing uses specialized equipment and standardized test procedures, ensuring that results are accurate, repeatable, and regulatory compliant. The data collected is vital for maintenance planning, safety assessments, and issuing operational notices (like NOTAMs). This glossary entry covers critical concepts, procedures, equipment types, and standards central to airport pavement friction testing and maintenance.

Key Terms and Concepts

Aircraft Braking Capability and Directional Control

Aircraft braking capability is the measure of an aircraft’s ability to decelerate safely on the runway. It depends on the friction between tires and pavement. Directional control is the ability to keep the aircraft aligned with the runway centerline. Both rely on the friction coefficient, or Mu value, at the tire/pavement interface.

When friction drops below defined thresholds—due to water, snow, ice, rubber deposits, or pavement wear—braking and steering effectiveness deteriorate, raising the risk of runway excursions. International standards (ICAO Annex 14, FAA AC 150/5320-12D) require airports to monitor and maintain minimum friction levels, triggering corrective action when limits are not met.

Airport Operators

Airport operators are responsible for the safe, compliant, and efficient operation of airport infrastructure. Their duties include scheduling and performing friction tests, maintaining equipment calibration, training staff, keeping records, and organizing corrective maintenance. Operators must act promptly if friction falls below safety thresholds, which may involve cleaning, rubber removal, or surface treatment. They are also responsible for issuing NOTAMs and coordinating with air traffic control when runway conditions change.

ASTM Standards (E2340, E1551, E524, E670)

ASTM International publishes technical standards for pavement friction measurement:

• ASTM E2340: Procedures for measuring pavement macrotexture with the Dynamic Friction Tester (DFT).
• ASTM E1551: Specifications for friction test tires.
• ASTM E524 & E670: Requirements for tires used with locked-wheel and side-force testers.

Compliance with these standards ensures that equipment, procedures, and results are consistent, repeatable, and accepted by regulators.

Braking Action Reports

Braking action reports provide real-time advisories about runway braking effectiveness. Pilots or operators categorize reports as “Good,” “Medium,” “Poor,” or “Nil” based on measured friction or pilot perception. These reports trigger unscheduled friction testing and may lead to runway restrictions or closures if friction is below operational thresholds. They also help validate maintenance actions and support safety management systems.

Continuous Friction Measuring Equipment (CFME)

CFME are specialized vehicles or devices that measure runway friction continuously over the entire length of the pavement. Equipped with standardized test tires and water application systems, they collect friction data at regular intervals, producing comprehensive profiles used for safety assessment, maintenance planning, and regulatory compliance.

Popular CFME devices include:

• Dynatest Runway Friction Tester
• ICC-DFT™
• GripTester
• Mu-Meter
• Skiddometer

CFME must be regularly calibrated and operated by trained personnel.

Calibration of Measuring Equipment

Calibration ensures that friction measuring devices provide accurate, reliable results. It involves verifying output against reference standards and adjusting as needed. Calibration covers water flow rates, tire inflation, sensor accuracy, and data logging. Detailed calibration records are required for regulatory audits and investigations.

Corrective Actions

When friction measurements drop below set thresholds, corrective actions restore safe runway conditions. Common methods include:

• Mechanical or high-pressure water removal of rubber deposits
• Chemical cleaning
• Surface texturing (grooving, shot blasting)
• Applying thin overlays or slurry seals

Corrective actions are dictated by the severity of friction loss, pavement type, and operational needs. Runways may be closed if friction is below minimum standards until restored and retested.

Contaminated Runway

A contaminated runway has more than 25% of its surface covered by water, ice, snow, sand, or rubber, significantly reducing available friction. This condition increases landing and takeoff distances and makes directional control difficult. Regulatory requirements include issuing NOTAMs, performing unscheduled friction testing, and removing contaminants.

Dry, Wet, and Slippery Wet Runway

• Dry runway: No visible moisture or deposits; highest friction.
• Wet runway: Thin water film (up to 3 mm); reduced friction but still operational if above maintenance thresholds.
• Slippery wet runway: Friction below safe levels due to surface wear, rubber, or polishing; requires immediate corrective action.

Runway condition affects operational procedures, landing distances, and maintenance scheduling.

Dynamic Friction Tester (DFT)

The DFT measures dynamic friction properties by rotating a disc with rubber pads against the pavement at set speeds and slip ratios. It produces friction-speed curves, helping identify surfaces prone to hydroplaning or excessive wear. The DFT is used alongside other texture measurement methods for a complete safety assessment.

FAA AC 150/5320-12D

This FAA Advisory Circular defines standards for measuring and maintaining skid-resistant airport pavement. It specifies approved equipment and methods, calibration, survey frequencies, minimum friction levels, and corrective procedures. Compliance is required for Part 139 airports and strongly recommended for commercial airports.

Friction Coefficient (Mu Value)

The Mu value quantifies the ratio of horizontal tractive force to vertical tire load. It is the primary metric for runway skid resistance. Regulatory authorities set design, maintenance, and minimum Mu values to guide maintenance and operational decisions. Values below 0.5 generally indicate unsafe conditions.

Friction Measuring Equipment

Friction measuring equipment includes CFME, Mu-Meter, GripTester, Runway Friction Tester, Skiddometer, and Surface Friction Tester. These devices must meet FAA, ICAO, and ASTM standards and be regularly calibrated.

Friction Survey Frequency

Survey frequency is based on daily turbojet landings, per FAA and ICAO guidance:

Unscheduled surveys are required after incidents or adverse weather.

GripTester

The GripTester is a CFME device measuring friction via a smooth test tire at set speeds and is equipped with a water system for wet testing. Widely accepted by aviation authorities, it is known for reliability and ease of use.

Hydroplaning

Hydroplaning occurs when water separates aircraft tires from the pavement, causing loss of braking and control. It is influenced by water depth, speed, tire pressure, and pavement texture. Friction testing under wet conditions helps identify and mitigate hydroplaning risks.

Operational and Regulatory Context

Friction testing is a regulatory requirement under ICAO Annex 14, FAA AC 150/5320-12D, and ASTM standards. It forms the foundation of runway safety management, maintenance planning, and operational decision-making. Data from friction testing guides corrective actions, informs NOTAMs, and ensures compliance with international aviation safety standards.

Airport operators must maintain a friction management program that includes regular testing, equipment calibration, staff training, documentation, and prompt response to friction loss. This comprehensive approach is essential for ensuring safe aircraft operations and minimizing runway incidents.

Summary

Friction testing of airport pavements is a complex but critical part of airport maintenance and safety management. By systematically measuring, monitoring, and maintaining runway friction, airports ensure safe landings and takeoffs, regulatory compliance, and the prevention of incidents such as skidding or hydroplaning. Ongoing advances in standards, equipment, and procedures continue to improve the effectiveness and reliability of friction testing worldwide.