Originally Posted by
Lead Balloon
And what 'clearance' would that give to a vehicle on the road below?
How long is a piece of string? If there's a 7.6m high vehicle, then none I guess. If it's 1.6 then 6m. You can do the math. As I said, I don't know if the road traffic is the obstacle. I'm just spit-balling some numbers based off the published gradient, and where the threshold is actually located. This all assumes also that the road is at the same elevation as the threshold. The 7.6m is measured at the road from the same elevation as the extended centre line.
The Approach Gradient used depends on a number of factors. RWY code, Instrument or not, Precision or not. The CBR master plan says that sometime in the last 20 years, RWY12/30 was reclassified (I'm assuming an improvement) to a Code 2B RWY. I don't have any idea what it was prior. Code 1 non-instrument RWYs have an approach gradient of 5% (For comparison, Code 1 instrument non-precision is 3.3% and precision is 2.5%). Code 2 NI should be 4%. Perhaps that is why 30 is said to use 5% now, because they couldn't meet the 4% when they reclassified it, or that is why the Threshold was displaced a few metres and still used 5%. Who knows what was involved in the reclassification.
The gradient is measured from 60m prior to the threshold, at the same elevation and splays out 10 degrees either side of the RWY strip, There should be no obstacle penetrating through it. If there is, the start of the gradient is moved until there is no intrusion, 60m is added, and there is your threshold. If you already have a threshold, and there is a new obstacle that penetrates and cannot be removed, then the threshold is displaced to compensate, leaving the existing RWY still behind it for take offs in the same direction and takeoffs and landings from the other direction.
If you fly the gradient to the threshold you will pass over the obstacle (if there is one) due to the 60m built in. It's also why the painted aiming points are further down the runway so that there is always a margin for error in the approach.