[no bug] HSI distance indicator goes to 9999nm?

[some1]

Not sure about that one, but usually these distance indicators go up to 999 nm, not 9999 nm, even in newer airframes like F-15. TACAN has reliable range below 200 nm and no one is flying 1000 nm legs on INS either.

Some drawings in the F-4 manual also make more sense if you read the last digit on the distance indicator as decimal point, so 9,5 nm instead of 95 nm.

[Dominik96]

Hi @some1!
The HSI in the F-4E is the AF/A24J-1 type. Here is a clip from the military specification describing the device (MIL-H-27269A). It clearly states the indicator shows whole (not decimal) miles up to 1999nm (so not that high as 9999nm).

Edited June 10, 2024 by Dominik96

[CF104]

5 hours ago, some1 said:

Not sure about that one, but usually these distance indicators go up to 999 nm, not 9999 nm, even in newer airframes like F-15. TACAN has reliable range below 200 nm and no one is flying 1000 nm legs on INS either.

Some drawings in the F-4 manual also make more sense if you read the last digit on the distance indicator as decimal point, so 9,5 nm instead of 95 nm.

 

Keep in mind that the HSI distance display doesn't only show TACAN range. With the BRG/DIST switch set to NAV/COMP it can display Nav Computer destination which can be much farther than a TACAN station. And it's possible to have a destination beyond 1000NM if you're crossing an ocean with several air refuelings.

Cheers,

John

Edited June 10, 2024 by CF104

[some1]

2 hours ago, Dominik96 said:

The HSI in the F-4E is the AF/A24J-1 type. Here is a clip from the military specification describing the device (MIL-H-27269A). It clearly states the indicator shows whole (not decimal) miles up to 1999nm (so not that high as 9999nm).

Thanks, makes sense if the leftmost digit is either 0 or 1. In the meantime, I found a diagram that also confirms what you wrote: 

http://aviation.watergeek.eu/images/f-4b/flight_director_group_7.1.5_hsi_schematics.jpg

Looks like they dropped this requirement for newer aircraft.

49 minutes ago, CF104 said:

And it's possible to have a destination beyond 1000NM if you're crossing an ocean with several air refuelings.

Destination yes, but individual route legs are usually much shorter than that, even in modern aircraft with latest equipment. I don't think you can even legally file a flightplan that has legs longer than 500 nm, though military aircraft obviously don't always have to.