fixed Autopilot Steering Select Seems Incorrect

[777coletrain]

Autopilot steering select seems to follow the same or similar logic that the tadpole used to. It ends up trying to home to a steer point instead of tracking directly to it. I’m not entirely sure this is wrong but I’ve personally never seen an autopilot system act this way.

F16 autopilot 1 16 22.trk

Edited January 16, 2022 by 777coletrain
Phrasing

[Frederf]

The INS steering should use the roll channel to null the track error. It looks at the subtraction of the INS track and course to destination and tries to make that zero. Heading has nothing to do with it.

[777coletrain]

18 hours ago, Frederf said:

The INS steering should use the roll channel to null the track error. It looks at the subtraction of the INS track and course to destination and tries to make that zero. Heading has nothing to do with it.

I agree, I'm currently trying to find any documentation on how this mode should work. However I find it hard to believe that it would just point the nose since that wouldn't actually bring you to the steering point when there is a lot of wind. If I'm wrong so be it but I would be surprised that the F-16 could input a GPS coordinate but not fly directly to that point. 

[777coletrain]

I've talked to a few people that have flown the F-16 and they've said that the flight path marker should point at the steer point. Please just ask the SMEs about this. 

[777coletrain]

Has there been any update with this?

[Falconeer]

On 1/19/2022 at 6:10 AM, 777coletrain said:

I've talked to a few people that have flown the F-16 and they've said that the flight path marker should point at the steer point. Please just ask the SMEs about this. 

If you put the flight path marker on the steerpoint, then yes. Otherwise its just the tadpole which points to the steerpoint or the steering carret on your compass.

Flightpath marker shows where the aircraft is going, not where the waypoints are located. If you'd put the flightpath marker on the ground, it will crash at that exact point

[777coletrain]

19 minutes ago, Falconeer said:

Flightpath marker shows where the aircraft is going, not where the waypoints are located

Correct, which is why when "steering select" is being used the flight path marker should end up over the steer point.  When there is a crosswind the aircraft currently doesn't actually go to the selected steer point because it simply flies the heading to it instead of the track.

[777coletrain]

The manual states "Positioning the ROLL switch to STRG SEL allows the autopilot to steer the aircraft to the selected steerpoint using roll commands. The roll command does not exceed a 30-degree bank angle or a 20-degree/second roll rate." The important part from this section is "to the selected steerpoint".

When an aircraft needs to go to a specific point on the ground (a steerpoint), crosswind will needed to be accounted for. This is because if the pilot (or autopilot) doesn't and points the nose of the aircraft at whatever they are flying to, the aircraft will home to that point. This is common in NDB navigation.  This is not preferable because it is inefficient and will require constant heading changes as the aircraft is constantly being pushed sideways. Currently this is what the STRG SEL switch does in the F-16.

If the pilot (or autopilot) was to actually fly "to the selected steerpoint" they would account for wind by setting a wind correction angel. I could go into how that is calculated and done in other aircraft, but effectively for fighter jets it would be accomplished by putting the flight path marker vertically above the steerpoint. By doing this the pilot (or autopilot) would account for any crosswind that exists and would actually fly "to the selected steerpoint".

I will include some pictures to illustrate what I've been talking about below. The bottom depiction is what STRG SEL is doing.

Edited February 12, 2022 by 777coletrain

[Frederf]

The INS keeps track of a quantity known as steering error which has nothing to do with heading. It's what drives the "W" symbol on the MFD, GCSC-FPM relative positioning, and lateral autopilot. In short, zeroing out the relative bearing is incorrect behavior. Zeroing out steering error is correct behavior.

[Deano87]

I’ve noticed this issue as well. 

[Mikaa]

21 hours ago, 777coletrain said:

The manual states "Positioning the ROLL switch to STRG SEL allows the autopilot to steer the aircraft to the selected steerpoint using roll commands. The roll command does not exceed a 30-degree bank angle or a 20-degree/second roll rate." The important part from this section is "to the selected steerpoint".

When an aircraft needs to go to a specific point on the ground (a steerpoint), crosswind will needed to be accounted for. This is because if the pilot (or autopilot) doesn't and points the nose of the aircraft at whatever they are flying to, the aircraft will home to that point. This is common in NDB navigation.  This is not preferable because it is inefficient and will require constant heading changes as the aircraft is constantly being pushed sideways. Currently this is what the STRG SEL switch does in the F-16.

If the pilot (or autopilot) was to actually fly "to the selected steerpoint" they would account for wind by setting a wind correction angel. I could go into how that is calculated and done in other aircraft, but effectively for fighter jets it would be accomplished by putting the flight path marker vertically above the steerpoint. By doing this the pilot (or autopilot) would account for any crosswind that exists and would actually fly "to the selected steerpoint".

I will include some pictures to illustrate what I've been talking about below. The bottom depiction is what STRG SEL is doing.

 

This. If the STRG SEL was in fact just a simple homing autopilot, it would never be used IRL due to significant wind drift, and pilots would just use the HDG function to track to the desired steerpoint by referencing the tadpole and vv cues and collocating them manually. Just because the F-16 autopilot is incapable of intercepting a specific HSI selected course to a steerpoint does not mean it doesn’t have the capability to compensate for wind drift and track (read steer) directly to the selected steerpoint. I really hope this is looked at again. 

[BIGNEWY]

Thanks the team will take another look, I have made a report.