Prerequisite: From Annex 14 (AERODROMES)….
Water on runway: Whenever water is present on a runway, a description of the runway surface conditions should be made available using the following terms:
- DAMP — the surface shows a change of color due to moisture.
- WET — the surface is soaked but there is no standing water.
- STANDING WATER — for aeroplane performance purposes, a runway where more than 25 per cent of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by water more than 3 mm deep.
Information that a runway or portion thereof may be slippery when wet shall be made available. Besides water on runway, friction coeeficient/level of the runway also plays a crucial part while deciding the slipperyness of the runway.
Now ICAO has introduced a new format called the Global Reporting Format of reporting water, slush on Runway. This will come into affect from NOVEMBER 2021.
Runway slippery when wet (When below minimum friction level – Decided by the State.)
- Design objective for new surface (DONS) – Mu meter trailer 0.72
- Maintenance planning level (MPL)- Mu meter trailer 0.52
- Minimum friction level (MFL)- Mu meter trailer 0.42
What is Aquaplaning/Hydroplaning – It is a condition in which standing water, slush or snow, causes the moving wheel of an aircraft to lose contact with the load bearing surface on which it is rolling, hence loosing required friction to stop within described limits which results in non effective braking action of the aircraft.
What Actually Happens – A layer of water builds up beneath the tyre of the aircraft which has a vertical force which progressively tends to lift the tyres above the load bearing surface, thi reduces the area in contact with the runway until the aircraft is completely water-borne.
Now in this critical situation neither the aircraft is able to make directional control nor effective braking action is achieved which may cause a severe accident.
Governing Priciple: This phenomenon is governed by a simple formula (Horne’s formula) to calculate the minimum groundspeed for initiation of this type of aquaplaning on a sufficiently wet runway based upon tyre pressure where V = groundspeed in knots and P = tyre inflation pressure in psi:
V = 9 * √P
Lets take P = 160 psi, will give V = 113.84 knots which means even relatively small aircraft may be subjected to this phenomenon. So it becomes a matter of grave concern during landing and takeoffs on wet runways.
Thus it is very important to maintain the runway surface conditions to an optimal level to prevent this phenomenon from occurring.