need track replay FPAS incorrectly calculates descent for TACANs & uses to steep a profile in general.

[norman99]

When descending using the FPAS calculated optimum descent, the end point is calculated correctly when a waypoint is the reference, but incorrectly when a TACAN is the reference.

As it displays the distance a descent should be commenced for your given altitude, if you keep the FPAs calculated number and your actual distance the same, you can fly and monitor the descent all the way down.

When using a waypoint, this works well, as shown below.

@30nm

@20nm

 

@9nm

 

@2nm (slight error is due to getting a little low compared with the profile)

 

When using a TACAN as the reference, the descent is not calculated correctly. As you can see below.

@30nm

 

@20nm

 

@10nm

 

@5nm

 

@2nm

 

@1nm

 

As you can see, the TACAN reference leaves you about 5,000ft too high. To illustrate more clearly, this is the two descent profiles compared:

Distance    -     Waypoint Alt    -     TACAN ALT

30nm                 ~30K                       ~33.5K

20nm                 ~20K                       ~23.5K

10nm                 ~10K                        ~14.7K

5nm                   ~5k                          ~10K

2nm                  ~1.5k                         ~7K

 

Secondly, the standard descent profile is, use idle power and descend at cruise Mach until 250kts, than 250kts onwards. ie M0.83-->250kts. This roughly correlates into a 2-to-1 descent profile for every 1,000ft, give or take 5nm. Eg, 20,000ft requires ~40nm to descend, 30,000ft 60nm etc. As you can see, this is almost double the distance calculated by FPAS when referencing a waypoint, and even more incorrect when referencing a TACAN.

I believe that FPAS descent figures are incorrect, and result in an overly steep descent. They also calculate the wrong final altitude when referencing a TACAN.

Edited April 13, 2022 by BIGNEWY
1.16

[BIGNEWY]

Hi, 

please include a track replay, and the images dont seem to be working for me. 

Please do not include NATOPS references as it breaks our 1.16 forum rule. 

thanks

[norman99]

Apologies for the NATOPS quote.

Not sure why the images aren't working for you. They appear on all my devices, but can take time to load.

I'll try to get a track attached as soon as I have the time.

[Frederf]

My understanding is that this is normal for TACAN profile to leave you high over the facility and is intentionally different than the waypoint logic. It's for a flameout landing or instrument approach or something like that. The idea is when you're coming back that low on fuel you want the assumed safety of entering in a high overhead pattern. F-16 does the same thing on the HOME profile, 5,000' over home plate, not 0.

The descent profile on the other hand I would think would be near the optimum minimum fuel trajectory. I don't understand why following that guidance would be so steep.

[norman99]

3 hours ago, Frederf said:

My understanding is that this is normal for TACAN profile to leave you high over the facility and is intentionally different than the waypoint logic. It's for a flameout landing or instrument approach or something like that. The idea is when you're coming back that low on fuel you want the assumed safety of entering in a high overhead pattern. F-16 does the same thing on the HOME profile, 5,000' over home plate, not 0.

The descent profile on the other hand I would think would be near the optimum minimum fuel trajectory. I don't understand why following that guidance would be so steep.

Is this intentionally high descent when referencing the TACAN documented anywhere? NATOPs implies there is no difference between a waypoint and a tacan station when calculating descent guidance. If it is implemented for one of the reasons you said, instrument approach, flame out (though less of a problem for the Hornet v Viper), I’d expect this to be explained somewhere.

Edited April 18, 2022 by norman99

[Frederf]

Everything is documented somewhere. I haven't seen it. The endpoint height being different source is the linked thread so take that for what it's worth. The similar idea in the other plane is from my memory bank. The guidance to any endpoint I would assume would be fuel saving but it could be tactical (kinda weird as a FPAS function).

[IvanK]

Warning Thread Drift :). Before the days of FPAS 2 numbers were in every Hornet pilots brain. Max range 4.2Alpha Max Endurance 5.6 Alpha.

When on  on the bones in a Bingo profile you flew 4.2Alpha. When the home base was in sight you flew a 4.2 degree descent profile at Idle thrust. So in fact waited till home base was at -4 degrees in the HUD then select Idle and descend at -4 degrees Flight Path angle. Not very sophisticated and didnt allow for winds but worked.

Edited April 18, 2022 by IvanK

[oldcrusty]

17 minutes ago, IvanK said:

Warning Thread Drift :). Before the days of FPAS 2 numbers were in every Hornet pilots brain. Max range 4.2Alpha Max Endurance 5.6 Alpha.

When on  on the bones in a Bingo profile you flew 4.2Alpha. When the home base was in sight you flew a 4.2 degree descent profile at Idle thrust. So in fact waited till home base was at -4 degrees in the HUD then select Idle and descend at -4 degrees Flight Path angle. Not very sophisticated and didnt allow for winds but worked.

 

Sounds like a good crutch to follow for land based ops, not only in good weather.  Hey, I use FPAS on every hop... selecting the last blank wpt as a home plate. 

[IvanK]

Looking at the relevant DOC for the FA18C Max range descent Idle thrust at 35,000lbs from 30,000ft gives descent dist of 60nm

[IvanK]

And Bingo charts give a Descent Flight Path angle of 4.5degrees for a clean jet.