If you multiply the distance to the threshold with 3, you will always get t to our height above the threshold (provided it's a 3 degree slope and you are on the slope). The fact that the airfield is high is not relevant. For example, airfield elevation is 1300 ft, at 10 nm you are at 3000 ft above the threshold. Another example; airfield elevation is 6200 ft, at 10 nm you are 3000 ft above the threshold. So you can see that the airport elevation does not have any effect. This trick can be used to figure out how high you are above airfield and use that to determine when to lower the gear can configure for final flaps. Normally you would lower the gear at 2000 ft above the airfield/threshold ( probided the speed is not fast), so that is always at 7 nm.
You could do that, but you have to set the speed bug higher than the current speed and if the aircraft captures the yoyo, it will add thrust if the speed bug is still higher than the current speed. This is a situation you have to watch out for and it increases your workload, so it is better to use OP DES with speed bug up, or VS with speed bug down, which has the same effect but you don't have to worry about the yoyo and unintended thrust increase. A lot of these things have to do with workload management. There are always multiple ways to do things but there is usually only one way which is less workload.
Can you explane more about how to lower the gear and final flaps and why its not relevent to the airport elevation . Ty so much Captain
If you multiply the distance to the threshold with 3, you will always get t to our height above the threshold (provided it's a 3 degree slope and you are on the slope). The fact that the airfield is high is not relevant.
For example, airfield elevation is 1300 ft, at 10 nm you are at 3000 ft above the threshold. Another example; airfield elevation is 6200 ft, at 10 nm you are 3000 ft above the threshold. So you can see that the airport elevation does not have any effect.
This trick can be used to figure out how high you are above airfield and use that to determine when to lower the gear can configure for final flaps. Normally you would lower the gear at 2000 ft above the airfield/threshold ( probided the speed is not fast), so that is always at 7 nm.
@7 to increase descent would it be preferable to keep DES mode with speed brakes instead of OP DES with higher speed or VS mode with lower speed?
You could do that, but you have to set the speed bug higher than the current speed and if the aircraft captures the yoyo, it will add thrust if the speed bug is still higher than the current speed. This is a situation you have to watch out for and it increases your workload, so it is better to use OP DES with speed bug up, or VS with speed bug down, which has the same effect but you don't have to worry about the yoyo and unintended thrust increase.
A lot of these things have to do with workload management. There are always multiple ways to do things but there is usually only one way which is less workload.
In the event of managed speed in open des& nav. Will speed constraints still be followed even if not in des?
Yes, as long as you are in NAV mode, the speed constraints will be followed if the speed is managed. No need to be in DES mode for that.