BOAC, Noted. However, the (old?) Boeing reference which I have been using does not appear to state that 15% is added - 737 QRH Sections 16 & 22. 737-400/CFM56-3_23.5K, June 2004. Quotes in blue.
Advisory Information - Normal Configuration Landing Distance
Tables are provided as advisory information for normal configuration landing distance on dry runways and slippery runways with good, medium, and poor reported braking action. These values are actual landing distances and do not include the 1.67 regulatory factor. Therefore, they cannot be used to determine the dispatch required landing field length.
However, irrespective of where 15% might be added, the landing distance achievable in normal operation may only at best equal the calculated landing distance (including 15%) due to the differences in the assumed touchdown point and threshold speed:-
Touchdown is normally considered to be 1000 ft from the runway threshold, but should be assumed to occur 2500 ft from the runway threshold when using autobrakes with autoland.
The problems of autobrake might only become significant with deteriorating runway conditions and can be masked by reverser use:-
Use of the autobrake system commands the airplane to a constant deceleration rate. In some conditions, such as a runway with “poor” braking action, the airplane may not be able to achieve these deceleration rates. In these cases, runway slope and inoperative reversers influence the stopping distance. Since it cannot be easily determined when this becomes a factor, it is conservative to add the effects of slope and inoperative reversers when using the autobrake system.
Using the tables as described by Boeing, tends to support the “we are OK to land syndrome” where the braking level is chosen to match the LDA with the advisory distance (no safety margin).
… enter the appropriate table for selected landing flaps and determine the reference landing distance for the selected braking configuration. Then read across the table to adjust the reference distance for landing weight, altitude, wind, slope, temperature, approach speed, and the number of operative thrust reversers, using the values provided, to obtain the actual landing distance.
A better method is to compare the distance margin provided by using higher brake levels and use the safety margin to counter the risks in the landing. In this instance, a tail wind is critical as it adds to the energy distance and tends to provoke long landings.
What should we do? Get ahead of the game.
Don’t get trapped in the rut of normality; every landing is different and should be assessed as such. The only normal issue is the quest for accuracy - altitude, speed, and distance.
Plan the landing with alternatives – what if. On a wet runway always check the landing data for a flooded runway (just in case it is reported or you assess it as such - heavy rain); brief the change in procedure required for any change in conditions (brake level, reverse). What’s the margin with spoiler / reverse failure?
Don’t trust PIREPS – would you bet the safety of your butt on the feeling of someone else’s butt – different aircraft, brake levels, weight, reverse, experience, risk taker, etc.
Think about how you are thinking – not “how we can do this”, but “should we be doing this”. Particularly for wet/flooded runways (heavy rain), tail/crosswinds, and heavy weight; beware combinations of these issues.
Edit:- and if the runway does not have a standard overrun area, deduct >90m from the LDA on a wet runway.
http://www.skybrary.aero/bookshelf/books/874.ppt
Last edited by PEI_3721; 1st Sep 2010 at 18:43.