Maintaining friction

In railroad engineering, the factor of adhesion of a locomotive is the weight on the driving wheels divided by the ‘starting tractive effort’. Adding extra power serves no purpose since there is not enough traction (adhesion, friction) to create useful work. The factor of adhesion which is 25% of the weight on drivers, corresponds to the co-efficiency of friction for steel-on-steel with an oxide surface film of μ = 0.25.

Within aviation we are constantly in search of the ‘stopping tractive effort’. The term ‘braking action’ has been used to describe this. It is not an accurate description since its use is not consistent. In the U.S. the term relates to the pilot reports only, within ICAO it has been used for both pilot reports and the reporting of runway surface conditions. A letter is due to be sent out to American States requesting feedback on the subject. In due course, this may result in changes across all levels.

We want to stop the aircraft in a controlled manner at the runway when executing an aborted takeoff or a landing. Of these two, the landing situation has challenged the aviation industry most and still continues to do so. As locomotives, the aircraft must provide traction under a wide range of conditions. The weight on braking wheels, the efficiency of the braking system and friction force available at the tyre/surface interface defines the stopping capability. In the aviation world the planning purpose steel-to-steel coefficient-of-friction, μ = 0.25, when measured by a ground friction measuring device on a compacted snow or ice surface. This would indicate a poor estimated surface friction using the table in the ICAO SNOWTAM format.

Accelerometer

The first instrument to measure the forces acting on aircrafts was an accelerometer. An accelerometer measures the acceleration and gravity it experiences. The readings are typically expressed m/sec2 or more popular in terms of g-force. The National Advisory Committee for Aeronautics (NACA) designed their own accelerometer in 1921. The extract below is from a NACA report from the time outlining the instrument’s other uses:

A longitudinal accelerometer would be a convenient method of measuring the pickup and braking power of automobiles, and a lateral accelerometer would record the side load on the tyres when rounding curves.

NACA Report No 100 – Accelerometer design, – 1921

Decelerometer

In 1925, Ferodo, a mainstay in British automotive industry’s brake system patented the Ferodo brake tester. This instrument, later known also as a Tapley Meter became much used and is still used to this day at airports. It was purpose built for the automotive sector to measure brake performance to within an industry agreed standard. Compacted snow and ice In 1949 an accelerometer was used for measuring the retardation of an aircraft and an automobile on the same winter contaminated snow/ice surfaces. The output was retardation (braking effect) expressed in m/sec2. This research was published in the ICAO Circular – Ice and Snow on Runways Report on the procedure for correction of Minimum Runway Length under Winter conditions at Oslo Airport, Fornebu. (1955) The accelerometer used was too sensitive and intricate. It was recommended to search for a more suitable measuring device.

In 1959, at an Inter-Nordic meeting, it was agreed that using a decelerometer (Tapley Meter) together with a table of comparative terms of braking action was the best way forward. This concept found its way in to what became the ICAO SNOWTAM format through IATA and UK proposals. Thus, on compacted snow and ice the consensus within the aviation industry in the 1960s was to use the dimensionless co-efficiency of friction and not the unit m/sec2. Furthermore, it was established that a decelerometer type measuring device should be the reference measuring principle (locked wheel) for the ICAO snow and ice table.

The same measuring principle is used by Canada when reporting the Canadian Runway Friction Index known as CRFI.

Aircraft brakes – efficiency of the braking system

The first aircrafts did not have brakes and consequently did not have sliding friction at the tyre/surface interface, although towards the end of the 1920’s brakes became more common. The main requirements for brakes for these early aircrafts were summed up as follows:

Powerful braking action without the tendency to turn the machine on its nose, low weight, smooth action, simplicity to allow for easy renewal of wearing parts, and the possibility of using the two brakes either together or independently to enable a machine to be turned on the ground at speeds so low that the air rudder is ineffective.

Flight (Flight International), March 22, 1928

With the development of hydraulic brakes for aircraft from the mid 1920’s the braking became somewhat comparable to braking of a car:

…and the pilot’s brake lever is then in direct communication with the brake shoes, so that the pilot can ‘feel’ accurately the amount of braking. The action of braking is then similar to that of a car.

Flight (Flight International), March 21, 1929

The next step was the invention in 1929 by Gabriel Voisin of the anti skid brake system (ABS). The efficiency of this system has had a significant influence on the achieved stopping capability. Today’s system is automated and can call for a target deceleration (auto brake setting). On a dry runway only the required amount of braking is needed. However, on runways that are not dry, the available friction force might not give the target deceleration. The amount of braking needed is then dependant upon the friction force available at the ‘tyre to ground’ interface and the efficiency of the braking system.

Ground friction measuring devices

The automotive industry developed ground friction measuring devices which date back to the 1920’s. Some aviation related research took place in the 1940’s but it was not until the 1950’s that purpose built ground friction measuring devices for the aviation industry were developed. As the number of measuring devices increased, it was inevitable that measuring principles would increase also. The four main principles in use at airports today are listed and discussed in detail in the EASA Runway friction characteristics measurement and aircraft braking (RuFAB) project reports (2010). They are:

• Locked-wheel testers
• Side-force testers
• Fixed-slip testers
• Variable-slip testers

These principles/devices were, at first mostly developed by various states independently and in a competitive manner. Crucially, the aviation industry did not agree upon one single measuring principle. The need for correlation emerged as different devices did not give the same readings at the same surfaces. ICAO was instrumental in producing a correlation programme in the early 1970s. As a result of this trial and other research, the ICAO Airworthiness Committee abandoned the plan of using ground friction measuring devices as part of defining a wet reference surface for aircraft certification purposes. Instead it was proposed that the devices could be used for maintenance purposes at Aerodromes. In the 1980s and 1990s a system was developed where ground friction measuring devices were used to identify a design objective for new surfaces, a maintenance planning level and a minimum friction level. These are commonly known through table A-1 in ICAO Annex 14 – Aerodromes, Volume 1, Attachment A.

A common misunderstanding is that those devices listed in this table have been approved by ICAO. This is not the case. Devices are approved at State level. Guidance is given in the ICAO Aerodrome Services Manual Part 2, and the State reports the approved information to ICAO.

Through the EASA RuFAB project, various shortcomings related to ground friction measuring devices has been identified. Poor device repeatability and device family reproducibility prohibits adequate harmonisation. None of the existing harmonisation models passed the project’s scrutinising. A possible future method was then outlined and proposed but not finalised. This means that at present, there is no consensus of a recognised method for harmonising ground friction measuring devices within the aviation industry. This is a serious problem, and only ‘best practices’ can be referred to. This implies that aerodromes who buy a ground friction measuring device should also buy into the ‘best practice’ of that State who approved the device.

What does that number mean?

The reading from a ground friction measuring device is a dimensionless number. Various design and proprietary algorithms are used by the manufacturers to produce this number. The basic idea is to control all factors belonging to the ground friction measuring device and then relate the readings to the surface friction characteristics of the pavement and the contaminants there upon. Today this is normally expressed as a coefficient of friction or, as in Canada, the Canadian Runway Friction Index (CRFI). One fundamental misunderstanding is that the number belongs to a scale from 0 to 1. This is not the case; the reading from a ground friction measuring device can be more than 1.

Another common mistake is that the reading (friction coefficient) is a property belonging to the surface. One should be aware of the fact that the friction coefficient is a dynamic system response and as such, mirrors all system variables that can influence the reading, including variables from the ground friction measuring device itself and the way it is handled.

What the number means depends upon the design of the device and the algorithms used.

ICAO Minimum Friction Level

There is a basic assumption behind the Minimum Friction Level (MFL) as defined by ICAO. Unfortunately this legislation is not readily available but hidden in documentation. When introduced it was essential that the MFL related to the operation of aircraft, without this relation it was said to be meaningless;

At the outset, it was agreed that the MFL of a wet runway would be that at which an aeroplane in the normal landing configuration using maximum wheel braking only would need twice the dry braked stopping distance. The MFL for each friction measuring device would, therefore, be such that an aeroplane stopping distance ratio (SDR) of about two was obtained.

ICAO Runway Surface Condition Study Group, Third Meeting, 1991

The method for determining the minimum friction level is described in ICAO Airport services Manual, Part 2, Pavement Surface Conditions, Appendix 1 and attempts a rational approach to the problem of defining MFL by equating the wet runway MFL to aeroplane wet landing performance, as defined by the Federal Aviation Administration (FAA) wet landing field length. There are two factors applied to the dry landing distance to arrive at these wet landing field lengths. The 1.67 factor which dates back to 1942 (CAR Part 61 – Scheduled air carriers rules), and the 1.15 factor which dates back to 1965 (FAR Part 121 Amdt. 121-9) Multiplied these two factors gives a stopping distance ratio of 1.92. Current research has questioned the safety level these factors implies, and thus the rationale behind the ICAO MFL and consequently the numbers attached to the various ground friction devices recognised in ICAO Annex 14, Vol 1, Attachment A.

The introduction of MFL into ICAO documentation represented a shift from the UK oriented approach to US standards.

The ultimate goal

The aviation industry is not in consensus of the ultimate goal of linking aircraft performance and readings from ground friction measuring devices. Some of the industry see this as unattainable, others are of the opinion that harmonisation and the enforcement of strict regulation of measuring devices is essential whilst others claim they have invented devices which will simply solve the problem.

The EASA (European Aviation Safety Authority) RuFAB reports show that there is still a way to go. EASA’s description of the RuFAB project points to the ICAO Friction Task Force as a regulatory body of influence:

Occurrences of runway overruns and veer-offs, where ice, snow, slush or standing water patches are contributing factors, have been a constant issue for aviation. The main objective of the research project is to provide a sound basis for the development of regulatory, standardisation or R&D actions, namely to contribute to the progress of the ICAO Friction Task Force (FTF) action plan and prepare the prerequisites to the future EASA rules for aerodrome safety.

http://easa.europa.eu/safety-andresearch/ research-projects/airports.php

ICAO Friction Task Force

The ICAO Friction Task Force is in the process of commencing its second phase. The recommendations from phase 1 (2008-2010) are now filtering through the ICAO system.

Part of the recommendation was not to report actual readings from ground friction measuring devices to pilots, this was due to the uncertainty attached to repeatability and reproducibility of the readings from the ground friction measuring devices.

In this second phase both interfaces between ground friction measuring devices and the operation of aircraft is on the agenda, namely through the search for a global reporting format and through an evaluation of the Minimum Friction Level. A single overarching source of guidance is needed for production and promulgation of runway condition information.

About the Author

Mr. Norheim has over 20 years of experience in management of airports with special focus on pavement surface conditions and reporting. He was the driving force behind establishing and running Avinors purpose built test track for evaluation of surface characteristics of asphalt surfaces and involved measuring techniques. He has establishied the real time touch screen reporting system used at Norwegian airports. Currently he is the leader of the ICAO Friction Task Force (FTF) who evaluate the related ICAO documentation and proposes changes and future activities.