Hello. Yesterday a friend asked me "If you're in a NDB approach, how do you know when to start the procedure turn?"
There's no table for the MAX TIME TO TURN and no DME station. Some VOR/NDB approaches require a procedure turn at 7 or 8 nautical miles from the station, so I've come up with a formula:
If we're flying at 130kts (130 nautical miles per hour) Ground Speed (GS), we divide it by 60. The result would be 2.166666 NM/minute.
If we have to start the procedure turn (PT) at 8NM from the station, we would have to divide it by 2.166666 NM/min. The answer: 3.69 (decimal) or 3:41 minutes (sexagesimal*).
8/(GS/60) and if you want to have an easier formula: (8x60)/GS. Replace the 8 with a 7 if the PT must be made at 7NM.
If the PT must be made at 8NM: 480/GS
If the PT must be made at 7NM: 420/GS
When flying at 100 kts GS and at 7NM a PT must be made, it is: 420/100=4.2 minutes, or 4:12 minutes.
Of course, we could have the different tables in our kneeboard:
If the airport we are arriving at has a DME, then our problems are solved :)
Next post will be a NACO (National Aeronautical Charting Office) chart explanation. Cya.
*Our results are decimal seconds. For a sexagesimal number, we must multiply it by 0.6 (Example: 0.79 minutes. 0.79x0.6=0:47 minutes)