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Old 5th Aug 2010, 22:35
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Skyjob
 
Join Date: Jan 2000
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An authorotive answer I cannot give you, but an educated one, yes:

- The first table in the PI is an energy reference chart; in other words, this is a table representing the energy the aircraft contains just prior to touching down (you could read kinetic energy); Vfly on top, Weight on left, the corrections for pressure altitude and temperature, plus some footnotes to correct for wind (1/2 HWC and 1+1/2 TWC) when planning in-flight or without a known groundspeed; if the actual Vtouchdown is known, use this speed and enter at 15 degrees (ISA) and Sea Level (0' Pressure Altitude)
- The second set of tables is a correction to this energy amount, to allow for the cummulatative friction during landing (ground friction, aerodynamic drag, slip ratio, etc), the selected auto brakes during landing and the reversers used. This corrected energy is that amount which is absorbed by the brakes during landing.
- The corrected energy is the amount that needs dissipating in some way or another, prior to departure, to allow for a safe aircraft starting the takeoff roll, in case of a high energy reject.

The entire calcualtion is very much so based on assumptions, such as the fact that brakes heat and cool evenly, which in reality cannot be the case.

The MQTW tables are there to enable the maximum possible weight to land with prior to reaching the Caution Area using the FCOM descibed landing technique. These weights are limitations be observed and the cooling time varies slightly by type variant, but for -800 is 67 minutes.

The Advisory data allows for different landing techniques (autobrake, reversers, less weights, ...) and less cooling time to achieve a safe departure sooner. In most airline operations you will never fly near the MQTW thus not usually need the full cooling time.

In an RTO, you enter the first table (kinetic energy) at sealevel with TOW and achieved speed during RTO. No corrections are made for reversers, autobrake, etc as it will go into full RTO braking (uncontrolled, maximum pressure available). Thus the uncorrected energy levels are identical to the corrected levels which then need refering to when cooling afterwards.

Looking at the tables you will see that the Reversers account for a lot of stopping capability and indeed are very capable of stopping the aircraft in good time with little use of any autobrake when required, thus low corrected values afterwards.

Remember that the lower autobrakes is selected, the lower the rate of deceleration, thus the longer it takes to stop from Vref, thus the longer the brakes are applied (be it less hard), thsu the longer time brakes are heated up. A downside to lower autobrake settings, but taken into accoutn in the tables.

Also remember that to release the brakes on stand improves the airflow around the brake stack, which is required to cool them down. No airflow, no or little cooling possibility!

More later, if required....
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