Mid Air Alignment

[bones1014]

Is it possible to perform a mid-air alignment in the Tomcat?

[electricaltill]

Yes, you can take a visual fix, use the radar or use a TACAN. The manual describes how.

[QuiGon]

Is it possible to perform a mid-air alignment in the Tomcat?

No, to the best of my knowledge, there is no option for inflight-alignment in the Tomcat, unlike there is in the Hornet or the Harrier.

 

Yes, you can take a visual fix, use the radar or use a TACAN. The manual describes how.

Uhm, what? That's a nav fix for updating the nav system, not an alignment to align the gyros, which is what the OP asks about. Totally different things! I recommend you read up on the difference between updating the nav system and aligning the gyros ;)

Edited August 29, 2019 by QuiGon

[electricaltill]

Uhm, what? That's a nav fix for updating the nav system, not an alignment to align the gyros, which is what the OP asks about. Totally different things! I recommend you read up on the difference between updating the nav system and aligning the gyros ;)

OP didn't mentio 'gyros', and I'm not familiar with the navigation systems of the F/A-18C or the Harrier. Doing a nav fix update makes navigation more accurate, which surely is the objective here.

[QuiGon]

OP didn't mentio 'gyros', and I'm not familiar with the navigation systems of the F/A-18C or the Harrier. Doing a nav fix update makes navigation more accurate, which surely is the objective here.

No it's not. Updating the nav system is not what the alignment does, which is what the OP asked about. The purpose of the alignment is to align the gyroscopes and accelerometers to the rotation of the earth, which is not what nav fix updates do. Some aircraft like the Hornet, Harrier or the A-10C have the ability to perform an inflight alignment, which requires the pilot to maintain a steady course and speed for the duration of the alignment.

Edited August 29, 2019 by QuiGon

[IronMike]

You have to have aligned on ground, in order to be able to perform a nav fix. What you cannot do, is take off without alignment and then start aligning in flight.

[bones1014]

Thanks for the input. I was flying RIO with a guy yesterday and aligning on the carrier didn't take so our system was all jacked up. I did some fixes but the INS didn't know where our nose was pointed.

[QuiGon]

Thanks for the input. I was flying RIO with a guy yesterday and aligning on the carrier didn't take so our system was all jacked up. I did some fixes but the INS didn't know where our nose was pointed.

Yeah, sounds about right. If the gyros and accelorometers aren't aligned, you can do as many nav fixes as you want to tell the INS where you're currently at, but it won't be able to make use of it as long as it doesn't know where up and down, left and right, forward and backwards is. That's why you need to do a proper alignment first, as you concluded correctly.

While you can't do an in-flight alignment with the Tomcat, you can do a manual (handset) alignment while still on the carrier, in case the automatic alignment doesn't work: http://www.heatblur.se/F-14Manual/general.html#handset-alignment

[Naquaii]

The F-14B currently in DCS is incapable of in-flight alignment due to the lack of a GPS and the fact that the INS is quite a bit older than the F/A-18C and AV-8B.

 

The updated F-14 with the CDNU and EGI has this ability as those add GPS and a more modern ring-laser gyro.

 

The reason for this limitation is that the computer controlling the gyro needs to be aware of the attitude and motion of the platform for the alignment to proceed. The earlier F-14 solves this by either being stationary, hence no movement, on a known position or by using the carrier's INS via the link.

 

In a modern system with in-flight alignment the computer uses the GPS for this information and due to the low rate of attitude data from the gps this usually requires the aircraft to fly straight for it to work.

[Robin_Hood]

Sorry to resurrect this post, but I have a question. What does the "IMU reset" procedure, described in the manual (page 282), do if it is not a kind of in-flight alignment? It does require to fly straight and level for 5 minute, which sounds like the sort of things you have to do for in-flight alignement.

Also, I've tried the procedure described (INS - OFF, then INS - IMU, fly straight and level for 5 minutes), but "IM" never appeared on the TID, it kept on displaying "AH".

[Naquaii]

13 hours ago, Robin_Hood said:

Sorry to resurrect this post, but I have a question. What does the "IMU reset" procedure, described in the manual (page 282), do if it is not a kind of in-flight alignment? It does require to fly straight and level for 5 minute, which sounds like the sort of things you have to do for in-flight alignement.

Also, I've tried the procedure described (INS - OFF, then INS - IMU, fly straight and level for 5 minutes), but "IM" never appeared on the TID, it kept on displaying "AH".

It's still not an alignment, those five minutes are to allow the gyro to stabilize after being turned off. If entered from the INS state this is normally not necessary as the gyro would've already be good to go.

But IMU/AM is a fallback nav mode for if you have failures in the INS that won't allow full functionality. In this mode the INS gyro isn't aligned, it's being used as free directional gyro mode so it will drift a lot over time. It's basically just used as an attitude reference and due to the heading axis not being stabilized by gravity it drifts a lot.

That the IM acronym isn't displayed could be for several different reasons but it could also be a bug, but I can't really say. You'd probably have to ask Grover who coded the INS for that.