TAM A320 crash at Congonhas, Brazil

[PEI_3721]

Re runway check (1mm water)
1mm may be safe on a good runway surface, but a simple check/measurement has little correlation with braking effectiveness on a poor or ungrooved surface. Furthermore, the definition of contaminated includes the % of runway covered, but this too does not guarantee that the aircraft will stop. All of these measurements and definitions are guidelines, generally in areas of operation that are technically ill-defined, and where the certification / operational regulations assume that the crew will compensate for the lower safety margins that the regulators allow, but rarely inform the operators about.

Many of the aspects in and around these problems are covered in Managing the Threats and Errors during Approach and Landing.
A good point made on slide 26 is in the comparison between wet grooved and ungrooved runways for the same speed. The comparison results in the complete range of ICAO reporting codes from ‘wet’ (Good, ICAO 5) to ‘contaminated / flooded’ (Nil, ICAO 1 – even ‘unreliable); thus 1mm could easily be nil/unreliable.

Perhaps we should treat any measurement on an ungrooved runway as ‘unreliable’.
Also be aware of concrete runways as these, either grooved or ungrooved, have poor braking conditions in comparison with well-drained tarmac.