FMS overlay approach where outbound leg is based on time
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FMS overlay approach where outbound leg is based on time
Helicopter pilot here! We in RW world are slowly moving into 21st century with fancy FMSs that can fly STARS and approches all on their own. However I think its fair to say we are still in our infancy when it comes to best practise and SOPs for using them. Therefore a question to you clever FW guys that have been doing it for years please:
NDB approach without DME which is basically a teardrop from overhead the beacon, outbound for 1min 30 secs before turning inbound. However FMS codes the end of the outbound leg as a fixed position, therefore depending on airspeed and wind, the inbound turn is started at a widely varying time after overhead. Not that it puts the aircraft outside the safe area of course, its just that the plate says "turn at 1:30 (still air)" and Mr CAA thinks we should follow the horizontal pattern on the plate, regardless of whether the outcome is safe and sensible or not.
Since as far as I can tell there is no ARINC 424 leg that terminates after a specified time, I can't help thinking that all FMSs must have the same issue and it must be one that you fixed-wing boys have had to tackle (probably many years ago). If so, please could you share the philosophy with me, and give me an idea how you check the plate against the FMS procedure when one is based on time and one on distance - and what criteria would you use to reject the FMS procedure in the database as being faulty.
Thanks in advance!
NDB approach without DME which is basically a teardrop from overhead the beacon, outbound for 1min 30 secs before turning inbound. However FMS codes the end of the outbound leg as a fixed position, therefore depending on airspeed and wind, the inbound turn is started at a widely varying time after overhead. Not that it puts the aircraft outside the safe area of course, its just that the plate says "turn at 1:30 (still air)" and Mr CAA thinks we should follow the horizontal pattern on the plate, regardless of whether the outcome is safe and sensible or not.
Since as far as I can tell there is no ARINC 424 leg that terminates after a specified time, I can't help thinking that all FMSs must have the same issue and it must be one that you fixed-wing boys have had to tackle (probably many years ago). If so, please could you share the philosophy with me, and give me an idea how you check the plate against the FMS procedure when one is based on time and one on distance - and what criteria would you use to reject the FMS procedure in the database as being faulty.
Thanks in advance!
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You will probably find that when the programmer was putting the rnav position into the database they looked at the the maximum speed for which the procedure was designed (aircraft category) and put the point at a distance equal to 1:30 at this speed.
Since you should make appropriate adjustments for wind effects on the timing, this position should be where you start your turn anyway (if you fly at the max speed used in constructing the procedure).
The CAA person is correct of they say that one has to comply with the procedure as published (horizontal picture) and the primary reference is still the raw data. However, the RNAV system is doing the donkey work for you by starting the turn at the ideal position every time no matter what the wind.
My recomendation - compare the timing 1:30 to the distance and the procedure speed and see if they are compatible.
Recomend that you contact the database provider and get them to explain in writing the method used to position the point and you can then give the CAA this info.
Since you should make appropriate adjustments for wind effects on the timing, this position should be where you start your turn anyway (if you fly at the max speed used in constructing the procedure).
The CAA person is correct of they say that one has to comply with the procedure as published (horizontal picture) and the primary reference is still the raw data. However, the RNAV system is doing the donkey work for you by starting the turn at the ideal position every time no matter what the wind.
My recomendation - compare the timing 1:30 to the distance and the procedure speed and see if they are compatible.
Recomend that you contact the database provider and get them to explain in writing the method used to position the point and you can then give the CAA this info.
DFC is correct. This would be a CF leg whereas a procedure turn is a PI leg, but in both cases the FMS has to have a fixed point at which to commence the turn. This will be based on still air at the maximum allowable speed for the a/c category - bearing in mind that generally they will be grouped A/B and C/D when the database is created. Some Databases will only offer C/D, so you may find that a procedure turn, for example, takes you out to 4.2nm before turning to intercept. It won't be a reciprocal either - just a continuous turn back to the end of the original CF leg (or CD leg if it was based on a dme range.)
Doc 8168 gives the ranges of speed allowable for the initial approch as:
Cat - A 90/150 (110 for racetrack or course reversals)
Cat B - 120/180 (140 for racetrack or course reversals
Cat C - 160/240
Cat D - 185/250
So you can see that if your database provider is a bit lazy sometimes (as is the case for the UNS-1) and only sticks in the C/D procedures the resulting legs may look a little 'odd' when you attempt to fly them Cat A!
Glad to see your steady progress into this sphere
Doc 8168 gives the ranges of speed allowable for the initial approch as:
Cat - A 90/150 (110 for racetrack or course reversals)
Cat B - 120/180 (140 for racetrack or course reversals
Cat C - 160/240
Cat D - 185/250
So you can see that if your database provider is a bit lazy sometimes (as is the case for the UNS-1) and only sticks in the C/D procedures the resulting legs may look a little 'odd' when you attempt to fly them Cat A!
Glad to see your steady progress into this sphere
