OverRun, I agree with your points about runway texture. The discussion re 3 mm water depth related to the TC report and common operational definitions; it was not intended to limit the discussion to this area.
A good research reference on the problem areas is
Wet Runways, but note that the information on antiskid systems and friction measurement devices have probably been overtaken by more recent information.
The operational problem might be seen as ‘how wet is a wet runway?’
In this domain, wet relates primarily to the water depth, but significantly includes runway texture, and tyre characteristics (fig 1 in the ref), noting that water depth need not necessarily be very great if either the texture and/or tyre characteristics are poor. It’s surprising how quickly we forget critical items, e.g. an 80% worn tyre is virtually ineffective in wet conditons – poor retardation and directional capability, yet in this commercially sensitive era we might allow tyre wear to the limit value.
Landing performance is published for wet conditions, but usually this is based on factored dry distances and crosschecked with computed / assumed friction levels with some measured distances.
ICAO ‘Good’ on a wet runway may be equivalent to 0.4 mu and above. The conditions are illustrated thus:-
Aircraft can expect to land comfortably within the scheduled ‘wet’ distance, without undue directional control problems, but this definition might now only apply to a well drained runway with average texture.
With increasing water depth / decreasing runway texture and poor tyre characteristics, the braking friction reduces and the estimated braking characteristics can range Medium to Poor. ATM should report this estimate which is often based on their view of the runway’s ‘wet’ condition, i.e. how wet is wet – how deep is the water in relationship to the texture.
However, ATM have little knowledge of the runway texture (except when NOTAM as slippery when wet) and no knowledge of the aircraft’s tyres, thus estimates of braking action can be very poor.
Braking action PIREPS are often quoted by ATM, but due to human limitations in assessment, particularly when thrust reverse and autobrake are used, these reports can result in highly misleading information as well as a source of peer pressure.
Thus, the pilot has the burden of the assessment and landing decision, but often lacking adequate information – how much water, what’s the runway texture, and what’s the condition of the tyres (should know, but who remembers).
The majority of current operations are on good runways (grooving/texture, drainage), which reduces opportunity to experience lower than expected friction (also a problem of reverse/autobrake use); this, and the reliance on thrust reverse, can lead to complacency when assessing the conditions for landing.
Factored landing distances provide a distance margin for maintaining a safe operation against the operational variability in normal operation (speed, height, position, brakes). The additional distance does not specifically consider variability in runway condition (although some reports suggest otherwise), and thus wet factors do not necessarily provide the same degree of safety as a dry runway.
As a wet runway becomes more wet (poor runway texture and tyres) the safety margin can vanish – greater reliance on reverse or the potential for an accident. In these latter conditions, pilots are expected to change their operation – reduce the variability of normal operations – use max braking, land on-speed, correct position, etc, but even with all of these, the unknowns in the runway condition may exceed all of the distance safety margin.
Another ref:
Effect of pavement texture.
… a pronounced surface texture effect and a large degradation in friction coefficient as a result of the addition of a small amount of water …