60% vs 1,67

[chr]

Hi
Recently I was asked by newbie what is commercial requirements to go for an airport regarding runway length.
So I proudly said 1,67 landing distance and you must land within 60% of LDA available , after that he asked me so which one of above datas need to be met : 1,67 or 60 % because those values are different. I started thinking then and really if you have runway 10000 ft long you need to be able to land within 60% of that lenght in regard to your AFM , so you need to be able to stop at 6000 ft .
But if you are taking your AFM calculations and you read there that you need 6000 ft than multipied by 1,67 gives you 10020 ft . It's of course a piece but I started to think why is that 1,67 instead of 1,60 to have those figures the same .

Thanks
Regards

[this is my username]

For the figures to be the same the multiplier would be 1.66666 recurring, aka 1.67 (or near enough).

[CL300]

Your reply should be : What is the number on your runway assesment ? Computation time is good for ATPL subjects :-)

[JonDyer]

The two numbers are not the same because they are different numbers.

60% of a 10,000 ft runway is 6000ft.

Your unfactored landing distance of say 3500ft x 1.67 gives you 5845 ft.

The 1.67 is 1/60 - in other words, how much runway do I need for 60% to be 3500ft?

If you have a runway of 5845 ft and you do your 60% calculation you'll find you just can just get in.

[tommoutrie]

the two numbers are not the same because they are different numbers...

classic..

You land within 60% of the available landing distance. Multiply the available landing distance by 0,6 to get the distance you need to be able to land within.
If you do it the other way round - that is you start by knowing the distance you are able to land in - you need to do the maths the other way round. So you divide the landing distance you need by 0.6. Dividing by 0.6 is the same as multiplying by 1.66666667 which is the same as multiplying by 10 and dividing by 6 and ....

oh my god I am my dad..

[safetypee]

The actual landing distance with a 1.67 factor, or higher (wet, etc), relates to the dispatch requirements for the planned destination.
Although not specifically required, it is good practice to use a similarly factored landing distance for the pre-landing assessment.

The 60% requirement relates to the actual landing distance vs the runway available, which for all practical purposes gives a similar factored distance as calculated for dispatch. However, recent recommendations to use actual plus a minimum or 15% as the baseline, then the 60% rule will require a longer runway.
Check the basis of the data in the manual; is it manufacturers actual (theoretical minimum), or actual plus recommended addition, or 1.67 actual ...
Even more recently manufacturers have been encouraged to publish Operational Landing Distances (OLD) which represent more realistically achievable 'actual' distances; these are often published with the 15% addition and become 'Factored OLD' = FOLD, which would be used for 60% rule.
Confusing, check the basis of the manual, the assumptions, and factors used.