View Full Version : Increasing CLEARWAY vs MTOW and V1

25th May 2003, 05:54
Could someone please answer what is probably a very simple question.......how does an increase in CLEARWAY affect
(1) your MTOW and (2) your V1???
I have asked several aviators, with varied levels of experience and have yet to get the same answer twice....they were all very convincing, but sadly no two answers were the same!!!

This makes me think that I am not the only one that does not know the correct answer!!!
Thanks for any help.

25th May 2003, 14:01
At the simplest level presume a baseline of balanced field length with no spare runway .. ie a critical length situation.

Daydreaming a bit .. if we were now to increase the weight a little on a BFL basis, V1 and both ASDR and TODR will increase and will exceed the available distances. However, if we now reduce the BFL V1 at the slightly higher weight, we can reduce the BFL ASDR at the expense of increasing the BFL TODR.

Provided that we address the TORR requirements, by having a clearway declared, we can take advantage of this V1 reduction to unbalance the takeoff (ie ASDR does not equal TODR) so that we can achieve the limiting ASDA while having the increased TODR contained within the clearway extension beyond the end of the runway.

As to whether the exercise is of any significant value will depend on the numbers for the particular runway, ambients, and the aircraft flight manual data... often the results are worth the effort in doing the extra sums.

25th May 2003, 14:31

Nice explanation:):):), just to put a value on the exercise, a B747-400 will get an extra 7,000 kgs out of Beirut (10,000 ft +) by using the 984 feet clearway.


Reverend Doctor Doug
25th May 2003, 19:56
Put simply, the performance engineers are allowed to include some clearway when calculating the performance figures. As clearway is not necesarily load bearing, it cannot be used for Acelerate Stop distances but it can be used for the go case.

Therefore the V1 can be calculated to allow the aircraft to accelerate on all engines to V1 and then stop by the end of the runway/stopway, or to continue from V1 on 1 engine and achieve 35' screen height by the end of the clearway, which may be further than the end of the stop way. There is a maximum amount of clearway that is allowed. From memory it is up to a maximum of 50% of the stopway. But don't quote me on that.

Cop u later

The Rev

26th May 2003, 11:58
Length of clearway ? - not quite.

Main limitation is an aircraft (AFM) one in that the takeoff run must be constrained to ensure that there is a low probability of the aircraft overruning the runway proper. For most rules, the maximum proportion of the airborne phase to screen height permitted over clearway is 50 percent.

The airport design standards give notional lengths for clearways but I am not aware that these are "limits" as such.

Alex Whittingham
27th May 2003, 20:15
There are, JT. Both ICAO and JAR limit the maximum clearway to 50% of TORA. Many AFMs have a further limitation on the amount of clearway that can be used to prevent the take-off calculation becoming TORA limited.

I reckon the extra clearway would reduce the minimum speed to go, VGO, this would increase MTOM and reduce V1. The explanation is graphical, I'll try to post the pictures later.

28th May 2003, 11:15

I tend to spend my time in the FARs. Could I trouble you for ICAO and JAR references to the limitation quoted so that I can review it for my familiarisation, please.

Thanks ... John T.

Alex Whittingham
28th May 2003, 16:55
Aaargh, I just knew you were going to ask that!

ICAO Annex 14 Volume 1 para 3.5.2
JAR OPS 1.490 (b)(2)

30th May 2003, 19:10
Ta, muchly, Alex ...