FlightDirector7 “would i be wrong in saying V1 is very conservative on the charts”
When you consider all of the variables then there may be no margin at all.
First to clarify some points in your post in case I misunderstood them: wet V1 is not the same as dry V1; you require more distance to stop on a wet runway. The decision to reject must be taken before V1, and the RTO first actions commenced at or before V1; the aircraft may not start decelerating until a later higher speed.
Some performance engineers (manufacturers) believe that the RTO distances are too close to the limits. This is particularly so when considering a wet runway.
As with landing performance, a single friction value associated with wet conditions is assumed in the RTO calculations. This value may not cover the wide range friction seen in service, e.g. increasing depth of water up to ‘contaminated’ (3mm), different surfaces – smooth vs grooved, concrete vs tarmac. Then there is paint and rubber deposits on the runway to consider, brake and tyre condition, and accuracy of wind information – the friction associated with the previous conditions are not know to the crew.
Where ever possible my SOP was to use the wet V1, I chose to accept a lower screen height in the rare event of an engine failure at V1 – go minded. This in my judgement entailed less risk than a stop from just before a ‘dry’ V1. The tricky problem is when the runway is wet (you need the wet V1), but would like to reject earlier – which is not possible if already at the Vmcg limit; be very, very sure about your stop decision – think about the situations (briefing), reasons, warnings, and actions carefully before take off.
The problem is heightened with poor/marginal runway conditions. Again comparing with landing; some runways are classified 'slippery when wet', where the operators are expected to take account of this; this warning (classification) is important during takeoff as it will effect an RTO – but who actually considers that?
Last edited by PEI_3721; 7th June 2008 at 16:25.