HSI TACAN Info - Any idea what this data is?

[-ORION-]

Thanks in advance!

[silverdevil]

the manual has this graphic. there seems to be more rows.

 

[Tholozor]

The data block underneath the TACAN identifier is the calculated fuel remaining once the aircraft reaches the station. The data block underneath that is the calculated range at which the aircraft should begin descent in nautical miles (assuming the station is at ground level).

 

The same applies to waypoints underneath the waypoint data blocks on the right when WYPT is boxed.

Edited February 18, 2021 by Tholozor

[-ORION-]

18 minutes ago, Tholozor said:

The data block underneath the TACAN identifier is the calculated fuel remaining once the aircraft reaches the station. The data block underneath that is the calculated range at which the aircraft should begin descent in nautical miles (assuming the station is at ground level).

 

The same applies to waypoints underneath the waypoint data blocks on the right when WYPT is boxed.

 

Great to know, thanks. Any idea why this isn't always visible? Sometimes I have it, sometimes i don't. Just need to know if it's me

[HILOK]

31 minutes ago, Tholozor said:

[...] The data block underneath that is the calculated range at which the aircraft should begin descent in nautical miles (assuming the station is at ground level).

 

is this assuming descent at current mach or ground speed, or in idle descent? do you know? thanks.

[Tholozor]

The NATOPS didn't specify if it's calculating a normal descent or maximum range descent. I would assume normal, which would be idle thrust, speedbrake retracted, and maintaining a descent speed of 250 KCAS, roughly 4000~6000 fpm (NOTE: This is according to the NFM-200 Performance Chart for the older GE-400 engines, the chart may be different in the NFM-210 and the GE-402 our Hornet has).

Edited February 18, 2021 by Tholozor

[Harker]

1 hour ago, -ORION- said:

Great to know, thanks. Any idea why this isn't always visible? Sometimes I have it, sometimes i don't. Just need to know if it's me

1. It's only visible if you have TCN or WYPT boxed.
2. FPAS calculations are only possible when you are at or below Mach 0.9. If you're going faster than Mach 0.9, the info goes away and you'll also get an FPAS advisory.

[Stearmandriver]

I believe you have to be heading somewhat towards the station as well.  Pretty tough to calculate fuel remaining without a positive closure. 

 

That said, sometimes the Tacan will glitch.  If it's not behaving as expected, turn it off and back on, that'll usually square it away. 

[Frederf]

It's bugged anyway. It should show the miles when descent should begin. Instead it shows a count down of miles to descent.

[Tholozor]

The thing there is that the NATOPS doesn't specify whether or not the range calculation stops once the aircraft begins the descent (unless you've got additional info not in the NATOPS).

 

At least it's being looked into from your initial report: 

 

Edited February 19, 2021 by Tholozor

[TimRobertsen]

As mentioned above:

The top number is calculated fuel remaining when you reach the tacan/waypoint.

The bottom number is at which distance you should start your descent.

 

You don’t have to be heading towards it.

 

As Harker said, above 0,9 Mach the FPAS-calculations are out the window, so you lose this data.

 

 

 

Regarding the question about descent-rate:

 

The number given is always very close to pr. 1.000 feet of altitude.

So if you are at 20.000 feet, the descent number will be close to 20.

At higher altitudes the descent-number will slightly deviate from this, but not very much.

 

So, according to trigonometry, there is a 1:1 ratio between 1000-feet-altitude and 1-nm of distance at a 9,46 degree descent.

 

In other words: if you are at 42.000 feet, the descent-number will usually be around 40. Then, at 40 nm (and still at 42.000 feet altitude) away from the tacan/waypoint you start a descent with 9-10 degrees down, you should hit the ground/water fairly close to the tacan/waypoint, within a couple of nautical miles

 

*This is for a tacan or waypoint at sealevel. I haven't checked this for a tacan/waypoint at altitude.

 

With Tacan/waypoint at altitude: the descent/number seems to aim for a 9-10 degree down too.

 

This is at least my best take on the descent-rate

Edited February 19, 2021 by TimRobertsen

[HILOK]

makes sense. -10deg pitch will also give you a decent (proper) speed during idle descent, which is the most economic performance-wise anyway.

[TimRobertsen]

Jupp, reagardless of loadout, one should be able to maintain a reasonable speed at 10 degrees down and idle

[Frederf]

10 hours ago, Tholozor said:

The thing there is that the NATOPS doesn't specify whether or not the range calculation stops once the aircraft begins the descent

It's been fixed. It used to count down to descent. E.g. you are 100 miles away and it would count down 70, 69, 68, 67, 66... as you approached. Now it is static 30, 30, 30, 30.

 

On the other topic it makes sense that the calculated range for descent will be constantly updated as the airplane changes altitude and it does in DCS. This is helpful to track your descent that you're on schedule and almost certainly how the real airplane functions.

[emibat]

On 2/19/2021 at 7:06 PM, TimRobertsen said:

 

With Tacan/waypoint at altitude: the descent/number seems to aim for a 9-10 degree down too.

 

I noticed that, putting a waypoint on the TACAN station, the descent point is different. At 9 ° on the waypoint I arrive on the ground, while from tacan, at 5000 ft (same rate).

[TimRobertsen]

16 hours ago, emibat said:

I noticed that, putting a waypoint on the TACAN station, the descent point is different. At 9 ° on the waypoint I arrive on the ground, while from tacan, at 5000 ft (same rate).

 

Jupp, there is something off with the descent-calculation for tacan. It might be a bug.

In Persia you end up at around 5900 feet above any given tacan, regardless of the tacans elevation, in caucasus this point is about 6600 feet.

I dont know if this is an actual feature of the Tacan system, or just a programming-error in dcs.

[TimRobertsen]

17 hours ago, emibat said:

I noticed that, putting a waypoint on the TACAN station, the descent point is different. At 9 ° on the waypoint I arrive on the ground, while from tacan, at 5000 ft (same rate).

 

Personally, I start a 10 degree descent when the distance (from tacan or waypoint) intersects the aircrafts altitude. So, if Im at 40.000 feet, I start a 10 degree descent at 40 nm distance.

 

If the tacan/waypoint is at an elevation, I subtract the elevation from the distance. For example an airfield at 2000 feet elevation, I start descent at 38nm.

 

If I want, f.ex, 3nm of level flight before I reach the point I add that to the range, 43nm of from the point.

 

This can be useful for when you approach for a carrier landing. You want to be at your given stack-level at 10nm from the carrier. At 40.000 feet alt, I’d start the descent at 50nm. If you have a stack-level of 3.000 feet, you could add that to the equation, but it’s not really necessary as you have start the descent that much earlier, the trigonometry of it is way off in that case  In short: you will arrive at favorable altitude at around 14-17nm off from the carrier, giving you plenty of time to adjust and prep for the stack  

 

It might seem like Im making a big fuzz about a simple issue of descent, but I thought it might be worth mentioning, in case someone would find it interesting

[LTRMcrew]

8 hours ago, TimRobertsen said:

 

Personally, I start a 10 degree descent when the distance (from tacan or waypoint) intersects the aircrafts altitude. So, if Im at 40.000 feet, I start a 10 degree descent at 40 nm distance.

 

If the tacan/waypoint is at an elevation, I subtract the elevation from the distance. For example an airfield at 2000 feet elevation, I start descent at 38nm.

 

If I want, f.ex, 3nm of level flight before I reach the point I add that to the range, 43nm of from the point.

 

This can be useful for when you approach for a carrier landing. You want to be at your given stack-level at 10nm from the carrier. At 40.000 feet alt, I’d start the descent at 50nm. If you have a stack-level of 3.000 feet, you could add that to the equation, but it’s not really necessary as you have start the descent that much earlier, the trigonometry of it is way off in that case  In short: you will arrive at favorable altitude at around 14-17nm off from the carrier, giving you plenty of time to adjust and prep for the stack  

 

It might seem like Im making a big fuzz about a simple issue of descent, but I thought it might be worth mentioning, in case someone would find it interesting

I certainly appreciate it! This has bugged me for some time and I think I naturally used the altitude  per distance 40k for 40 miles but I often descended quicker than 10 degrees. I will try and do this going forward. Thanks for the research and posts!

[TimRobertsen]

20 hours ago, LTRMcrew said:

I certainly appreciate it! This has bugged me for some time and I think I naturally used the altitude  per distance 40k for 40 miles but I often descended quicker than 10 degrees. I will try and do this going forward. Thanks for the research and posts!

 

I take back my previous statement about omitting the stack-level from the equation on the carrier approach/setting up for the stack  

 

When approaching a carrier from 40.000 feet, for the 10NM/See you at 10-point. If you have Stack at Angels 3, f.ex.

You can simply subtract 3 from the range, and add the 10NM, giving you a descent-go at (40-3+10=) 47NM.

If you “cradle” the Velocity Vector in the 10-degree ladder-step, like this.

This seems to work, pretty much, spot-on for a good stack-setup.

 

I know I keep going on with this, but I’ve gotten a little bit carried away this

[norman99]

Is this feature considered working? I don't seem to ever get a number that makes any sense.

 

In this screenshot, I'm at 7,500ft, at 38nm, and it shows my TOD at 2nm. shouldn't it be in somewhere around 7-8nm?

 

 

[TimRobertsen]

The Tacan Descent Guidance is a little different from the Waypoint Descent Guidance.

The Tacan aims for an altitude of around 5.900-6.500 feet above the tacan (depending on the map).

 

This makes it somewhat useless (Im stepping thin ice now:p) if you are heading towards a carrier, and want/need to be at your stack-level at 10NM from the carrier; or an airfield which is at a significant elevation.

 

This is apparantly how it actually works, I dont know why.. as it seems a bit wonky.

 

 

I always go for the "manual-descent-guidance" I decribed above, it is pretty much spot on in any scenario, you just have to do a little math

 

Edited March 6, 2021 by TimRobertsen

[norman99]

"Seems a bit wonky" is absolutely right. The NATOPS flight manual makes no mention of a descent target of ~6,000ft for TACANs. The only thing it does mention is if it is unable to calculate a descent, invalid Xs are displayed.

 

Hopefully ED can have another look at this.

 

Cheers

 

[TimRobertsen]

Jupp, Natops is a little short on explaining a bunch of stuff. We need more manuals! 

This with the tacans might be a part of general military aviation, and therefore not covered in the F/A-18's operating maunal. Could be :p

 

This has been reported, and ED claims that this is how the Tacan decent guidance actually is set up in the real Hornet... only the gods, old and new, knows why

[Frederf]

If CV TACAN is used does this descent guidance align with the Case III, CV-1 approach marshal hold? Maybe shore-based ops are sacrificed to harmonize with ship-based ops when it comes to FPAS TCN descent guidance.

 

The ship is moving and hold is what, 6000' at 21nm. Hmm, that doesn't agree either. I asked straight out "TACAN and waypoint FPAS descent are different, is this bug?" They said no. But perhaps SME said "yeah FPAS TACAN guidance minimum altitude is 6000" and ED assumed that meant 6000' at 0nm instead of 6000' at 21nm. Or maybe not. Maybe there is a special overhead marshal or maybe it's assumed clear weather instead. Since FPAS is giving you minimum fuel solution I could see it geared for situation where fuel is tight even if that is the rare situation. Maybe overhead at 6000' is a great place to be so control has time to manage your arrival.

 

I just don't know how the current FPAS TACAN descent profile would be used practically. When you begin your descent that extra ~5nm closer, what happens next?

[TimRobertsen]

Yeah, I just dont know man

 

Case III (and II to some degree) is a fairly strick process, I just dont see how it fits in.

 

My guess is that the Tacan-system, in a general terms, is a predefined concept within military aviation, and is just set up to put you at 6k above the tacan. But Im probably missing something 

 

There must be some pilots out here who knows the answer to this