- Thu Dec 20, 2018 2:09 am
Dhruv Kalra 878508 wrote:
Rob Nabieszko 1138610 wrote:While RVR is a useful information tool, it is not located on the runway, where the visibility matters. Although unlikely, there could be variations from the runway to where the RVR equipment is located.
I’m sorry, but what? RVR, the equipment that measures Runway Visual Range, isn’t located on the runway? Unless you’re referring to a scenario where you’d be on a non-RVR Runway at an airport where not all runways have RVR equipment, that statement is categorically incorrect.
It is important to understand exactly how RVR is designed to be able to apply the information it provides in a useful manner. A sensor measures the incoming light from a light located a fixed distance away, and translates this to a distance the pilot should be able to see. But like any other instrument, that value is only valid for the immediate area being measured.
It is truly important to consider WHERE the RVR sensor is mounted, relative to our departure runway.
Laterally: They are not located on the runway. Not even close. The FAA requires them to be mounted (at least) 400 feet laterally away from the runway centreline. I was unable to find a TC/ICAO/EASA reference, but from personal observation, they are several hundred feet from the centreline. (The sensor is normally 14 feet tall - you wouldn't want it too close to the runway.)
Longitudinally: Threshold and rollout installations in the US (again, the only docs a cursory google search revealed) are required to be installed within 2500 feet of each runway threshold. If you are departing from one of the displaced thresholds that started this whole thread, add the threshold displacement to that 2500. You could be a half mile or more from the RVR sensor.
The point is that the only truly accurate measurement of the visibility on the runway is the pilot looking at the runway.
Don't get me wrong, RVR is a fantastic tool. It gives us a great idea of what to expect when we get to the runway, whether it is even worth leaving the gate. But I have seen several occasions in my career where the RVR values across an airport have been completely different. 800 at one end, and 6000 at the other. The RVR value should always be taken with a grain of salt, and not regarded as gospel truth.
And counting runway lights in Canada during RVOP/LVOP is the standard all pilots operate by, or at least what is expected of all the operators I have dealt with.
Would you cross a runway without looking left and right, just because the controller cleared you to? Would you cross a street just because the light turned green? Are you willing to bet your life that every car will stop at the red light? Just another layer of increasing safety.