Why did the accident investigators not answer the obvious question, poor braking?

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Runway excursion is aviation’s number 1– Safety Risk Category. Among the top contributing factors are poor braking action due to contaminated runways combined with shortfalls in the accuracy and timeliness of assessment and reporting of the runway surface conditions. The accident at Kozhikode on 7th Aug 20 was a runway excursion. A few days after the accident I had written a blog on why the investigators must look at Hydroplaning & analyze the role played in the causation of the accident.

Even though the final report has ruled out Hydroplaning as one of the causes, the investigators have not provided a reason for the poor braking action of the aircraft immediately after touchdown.

Did Viscous Hydroplaning and the undulating yet smooth runway surface play a part in increasing the landing roll that fateful day?

Did the pilot release brakes and simultaneously stow the engine reverse thrust before selecting them back to break out of hydroplaning?

What is Hydroplaning?

When an aircraft tire rolls on a wet runway surface, water is squeezed out from the tire pavement contact patch and this process generates water uplift forces from underneath the tire. At a critical landing speed, the aircraft tire can get completely separated from the runway surface by a thin film of water, triggering a phenomenon known as hydroplaning. The critical speed at which the tire is completely separated from the pavement is known as the hydroplaning speed. Hydroplaning is a critical aviation safety issue since it results in near-zero friction coefficient between the aircraft tire and the runway pavement, and a loss of steering control of the aircraft, and can potentially lead to runway overruns. There are 3 types of hydroplaning, Viscous, Dynamic & Reverted Rubber.

Effect of forward speed on the tire-ground contact area in wet conditions

In zone 1 much of the water is ejected as spray and squeezed through the tire’s tread and the runway texture. Hydroplaning in zone 1 is the result of the hydrodynamic forces developed when a tire rolls on a water-covered surface. 

Zone 2 is a transition region. There is only a thin film of water in this zone and water pressure is maintained by viscous effects (hence the name viscous hydroplaning). Viscous hydroplaning typically occurs on wet/flooded runways that have a smooth micro-texture. Micro-texture is the sandpaper-like roughness of a surface formed by the sharpness of the fine-grain particles on the individual stone particles of the surface 

Zone 3 is a region of a dry contact. In this zone, the friction forces on the tire are generated when the wheel is braked or yawed. The braking friction force is approximately equal to the dry runway friction force multiplied by the ratio of the contact area in zone 3 and the overall tire-ground contract area. Therefore the smaller zone 3 gets, the lower the braking friction forces become.

The Flight Data Recorder (FDR) does not record the wheel spin (speed). Therefore the parameter has to be correlated with another dependent parameter. The speed brake was deployed 2 seconds after touchdown thereby confirming that the wheel speed has crossed 60kts and that the logic is:

  • Both throttle angles are less than 44 degrees (Reference: 36 degrees corresponds to forward idle)
  • Sensed wheel speed is greater than 60 kt on at least one even numbered and at least one odd numbered wheel
  • Speed brake lever is NOT in the DOWN position

Viscous Hydroplaning

This occurs when the runway surface is lubricated by a thin film of water, reducing the coefficient of friction. The tire is unable to penetrate this film and contact with the surface is partially lost. Essentially, it makes the runway slippery. When operating on damp or wet runways, a loss of tire braking and cornering ability is mainly attributable to this type of aquaplaning. However, it is most severe on runways with a smooth texture, where a layer of water only 0.01 inches (0.254 mm) deep can significantly reduce the coefficient of friction. This can occur at any tire speed. Generally, there is little or no post-event evidence available to determine if viscous aquaplaning has occurred.(ATSB – AO-2015-046 PP35)

FDR readout VT-AXH

Unlike Dynamic & Reverted Rubber hydroplaning, in Viscous Hydroplaning the wheels continue to spin but at a slower speed. The final report states that during inspection no physical signs of reverted rubber hydroplaning like, skid burn, tread rubber reversion marks and tread flat spots were observed on any of the tires. Analysis of the event of speed brake lights being recorded in the DFDR was carried out. It was inferred from the behaviour of the speed brake light that multiple wheels (at least one even number wheel and at least one odd number wheel) were spinning during rollout. Therefore, it was concluded that during braking, the tires had adequate contact with the runway surface and continued spinning above 60 kt. The accident final report concludes that the Wheel Spin is an indication of adequate contact of the tires with the runway and indicates that there was no hydroplaning but failed to consider Viscous Hydroplaning that can occur with the wheel spinning too.

Runway friction & Aircraft Braking Coefficient

An important source of information used by pilots (especially when assessing landing performance), is the runway braking action. For many years, extensive research has been conducted on runway friction. The establishment of a correlation between friction values measured using some kind of friction tester and that of an actual aircraft was the main objective in these studies.

The studies conducted so far, show that the correlation of the braking friction between a test device and an aircraft varies from good to poor correlation. This is caused by the fact that the aircraft’s tire and operational characteristics (such as tire pressure and ground speed) differ significantly from the friction testers. It is therefore not surprising that Boeing claims that there is no relation between friction testers and aircraft performance on wet and contaminated runways.

The braking action on a runway can also be based on pilot reports. However, studies have shown that there is no correlation between pilots reports and actual friction values of a runway. Intensive research on the subject of runway friction testers is still continuing.

( Safety aspects of aircraft performance on wet and contaminated runways G.W.H. van Es, A.L.C. Roelen, E.A.C. Kruijsen and M.K.H. Giesberts)

Aircraft Coefficient of braking

The airplane coefficient of braking as given in the final report was as follows:

  • Touchdown (4438′) till 7100′ Average .07mu, (the rolling friction coefficient on un-braked tires is about 0.02)
  • 7100′ till end of runway Average .14mu
  • Beyond end of runway (RESA concrete & soft ground) Rapid increase.
The grade of the aircraft braking coefficient and ground friction coefficient.

Deceleration devices

After touchdown, the Pilot Flying immediately applied maximum braking thereby disconnecting Auto Brake 3. Speed brake was activated after 2 seconds.

The thrust reversers were selected to deploy 03 seconds after touchdown and deployed 05 seconds after touchdown for a brief period of only 02 seconds only before being stowed back. 09 seconds after a touchdown during the process of stowing the thrust reversers, the aircraft brake pressure was momentarily reduced by the PF. The thrust reversers were deployed again 15 seconds after touchdown when the aircraft was near the end of the runway and max reverse thrust was selected. The engines began to spool up following the maximum reverse thrust command. After being deployed for 07 seconds, the thrust reversers were again stowed back, with the engines still being at a high fan speed (N1).

A braking Coefficient of 0.7 mu during the first phase till 7100′ of runway corresponds to ‘POOR’ braking. Since Speed Brakes were deployed after landing, it can be assumed that the wheel speed was above 60kts and there was no dynamic braking. The evidence against dynamic hydroplaning combined with the extremely low wheel braking friction coefficient that was achieved suggests that the airplane experienced viscous hydroplaning during the landing roll. Viscous hydroplaning is associated with the buildup of water pressure under the tire due to viscosity in a thin film of water between a portion of the tire footprint and the runway surface. On a wet runway, the maximum wheel braking friction coefficient decreases with increasing speed due to viscous hydroplaning. Therefore, despite the crew applying hard braking, the amount of water on the runway along with the possibility of viscous aquaplaning resulted in a reduced braking capability of the aircraft.

Runway profile & quality of construction

The aircraft touchdown was close to the crown of the runway (highest point) after which there was a downward slope. The uneven surface of the runway can be seen in the Google Earth elevation profile of the Runway. The undulations are also catchment areas for water that are deeper than 3mm thereby rendering the runway Contaminated with Poor braking action.

Google elevation profile depicting the down slope and undulating runway
Normal Load Factor due to the undulating runway
Water catchment areas

Conclusion

The only way a safe outcome was possible was if the braking action prevailing under the environmental and runway surface condition would have produced braking friction of .15mu or better. As per scenario 4 of the investigators when using aircraft braking coefficient as an average 0.15mu the distance to stop as calculated was 4849ft. The aircraft would then have stopped at the edge of the runway at 9287 ft. The pilot possibly realized that the aircraft is not decelerating as per the expected rate and is hydroplaning. In order to restore braking action, the pilot may have released the pressure on the brakes and canceled the thrust reverser momentarily before deploying them again. with this action the braking coefficient did improve by the lost ground during the 1st phase of the deceleration could not be offset and the aircraft overshot the runway.

While the final report has neither considered Viscous hydroplaning nor has it considered the condition of the runway surface which has the potential to become a catchment area for water making it contaminated. Friction tests carried out by the airport operator have been carried out on dry runways with the spray mechanism of the friction measuring vehicle. ICAO the UN body in its document for investigation recommends that the friction should be measured under similar conditions that of the accident, which means under conditions of light rain.

FAA has issued a Safety Order 19003 for Turbojet Braking Performance on wet runways. When planning to land on a smooth runway under conditions of moderate or heavy rain, pilots should consider that the surface may be contaminated with water of depth more than 1/8 inches and adjust their landing distance assessment accordingly.

In a rush to absolve the airport operator for not upholding safety standards, key actions and analysis have not been carried out by the accident investigation bureau.

Similar accidents attributed to Viscous Hydroplaning:

 Performance Margins Paul Schmid Boeing Aerodynamics Engineering

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