Ex Douglas Driver,
I see here a thinking individual who is a little concerned about 1st segment considerations within the Australian system of RDS distances and gradients. A very valid concern. In the creation of RTOWs we typically use the Type ‘A’ runway charts and survey maps for the areas beyond the Type ‘A’ areas, and sometimes STODs and TODA to create obstacle polygons (Not my original idea, I have
John_Tullamarine to thank for that gem). In so doing, we can assess the 1st segment obstacle clearance against ‘real’ data for real obstacles at real distances. As you have pointed out, when the RTOW is invalidated for any reason, a ‘fall back’ system is required for the pilot to calculate his own revised RTOW from TODA and STODs, and discrete obstacle data is not available.
The means that I use (and I’m sure that there are others), is to provide data for reduction of TODA and STODs to ensure that the 1st segment always lies above the runway and the obstacle-clear gradient for the STOD under consideration. What I describe here was approved by CASA for the operations for which I do P/E work, you will need approval......
This will not be a small project to use for a ‘one-off’ occasion, the work is far too much for that. Once produced, the table may be applied at any time that the RTOW is invalidated and you have to revert to the published RDS data. Whilst producing an obstacle polygon will give you the best performance, this data is runway specific and time consuming, and you will be far better served to develop a system which may be applied generically for all runways. You will need 4 essential items of information, i.e.
(1) The Runway Slope (RDS),
(2) The STODs and / or TODA Obstacle-Clear Gradients (RDS),
(3) The horizontal length of the 1st segment (AFM),
(4) The greatest difference between 1st and 2nd segment gradients for all Weights, Temperatures, and Pressure Heights (AFM).
Item (4) is the one that will be very time consuming, but you will find after the first few hundred 1st Vs 2nd segment gradient comparisons, that the difference is quite constant. Obtain the largest gradient delta as your base figure, this will cover your worst case scenario, so it can be used in all cases. Let’s call this gradient differential ‘DELTA’. Convert DELTA to degrees, where the angle = ATN(Gradient/100). Let’s call this angle ‘A’.
Next, from the AFM, find the 1st Segment horizontal distance. This will be quite constant even with weight variation, as it is based upon time of gear retraction, and the distance covered during retraction. If the AFM does not give you this, go to the ‘Close in Obstacle’ table, and extract the distance from reference zero to the end of the 1st segment (the point where the line suddenly bends upwards). Let’s call this distance S1D.
Now, subtract DELTA from the STOD Gradient to be used (1.6%, 1.9%, 2.2% etc.). This is the ‘safe’ 1st segment gradient to cover all cases. Let’s call it S1G.
IMPORTANT – As part of the 1st segment will now be flown above the runway, S1G must equal or exceed the runway slope. If it is less than the Slope, calculate the limiting weight for the runway slope as the 1st segment weight limit, and don’t worry about the 2nd segment as the actual gradient will exceed it. e.g. on Melbourne RWY 34 where the slope is +0.9%, calculate the 1st segment limiting weight for +0.9% or more (I ensure a minimum of 0.4% 1st segment gradient in my work, thus, on Melbourne RWY 34 I would ensure a 1.3% 1st segment gradient capability). Back to the main story……
Now, find the gradient and angle that the 1st segment achieves above the runway from the formula : S1G minus SLOPE, where Down Slope is Negative, and Up Slope is Positive. Convert this to degrees, and let’s call this angle S1A.
Now, we have 3 important values to compute, namely :
S1A : The Angle that the 1st Segment makes above the runway,
S1D : The horizontal length of the 1st Segment, and
A : The angular difference for the worst case comparison between 1st and 2nd segment angles.
Now, it’s a straight-forward calculation to find the REDUCTION in STOD or TODA length to ensure that the 1st segment remains above both the runway and the Obstacle-Clear Gradient :
REDUCTION = S1D X Sin A / Sin (180 – A – S1A)
Now, repeat it again and again and create a Runway Slope / OCG table for use with any set of RDS data.
Easy isn’t it? Now that you have the reduction, subtract it from the TODA or STOD, and, as you’re using Balanced Field, if the answer is greater than TORA, use the TORA.
Mutt would do that, but I suspect that
John_Tullamarine and I would use up to TORA+60M if it was available.
An Example :
You’re working with a 2.5% STOD of 1500M. Runway Slope = + 0.3% (UP)
Maximum difference between 1st and 2nd segment gradients in all AFM data is 0.9%.
1st Segment Distance (S1D) = 700 M
DELTA = 0.9 : Angle ‘
A’ = ATN (0.9/100) = 0.515648°
For a 2.5% STOD,
S1G = 2.5 – 0.9 = 1.6%
Gradient to Runway =
S1G minus
SLOPE = 1.6 – 0.3 = 1.3% :
S1A = 0.744803°
Substituting into the formula
REDUCTION = S1D X Sin A / Sin (180 – A – S1A),
REDUCTION = 700 X Sin 0.515648° / Sin (180° - 0.515648° - 0.744803°) = 286.4 M
Useable STOD Distance = 1500 M – 286.4 M = 1213.6 M
A piece of cake.
Hey
John_Tullamarine, you worked for XXXX airlines also? So did I, along with about 7,631 other Prooners who wish to remain anonymous. Ummm, there is one chess piece embedded here
Regards,
Old Smokey