Entering heading instead of Magvar leads to uncontrolled groundspeed indication.

[Looney]

As a RIO, I was happily starting up the bird from the backseat. Entered own aircraft lat/long and elevation, set INS to GND Align and waited.

 

After that, as we fond out in later testing, I thought I entered Magvar but accidentally entered own aircraft heading to 62 instead of selecting the proper CAP button Magvar.

 

What happens is that the groundspeed of own aircraft in the TID (when hooked to own aircraft and pressing CAP 8/Heading button) as well as the ECMD page starts to rise gradually. This happens when we were fully aligned, on the parking brake on the Apron at Kutaisi airport.

 

By the time we were about to take-off I noticed that HomeBase waypoint was 9 miles off and did a TACAN fix to update position. This worked as intended but due to the groundspeed ever increasing (even though TAS said 0 knots), I eventually lost all datalink and waypoint contacts because we were doing 3600 knots going to Greenland and were well outside of the max TID range of 400Nm.

 

In the end, the INS thought we were 15000 miles away from HB, travelling at 3600 knots.

The pilot noted the pitch ladder was angled and at a certain point, couldn't use the ladder at all.

 

We ruled out version difference between pilot and RIO, thinking of desync issues. We tested again on Kutaisi, same server, same aircraft but another pilot.

 

Steps to repeat:

 

1. Start the aircraft as normal.

2. Enter Lat/Long as per kneeboard

3. Enter elevation as per kneeboard.

4. Wait for full alignment

5. Enter a heading of 62 via the numerical keys of the CAP using the buttons 8 (heading), 6, 2, enter

6. set HB waypoint on CAP

7. Set HB position via CAP numerical panel.

8. watch ECMD GS go up

9. Hook own aircraft and select CAP numerical 3 (speed) and watch this go up

10. Note on ECMD that TAS is 0

 

What I did to remedey the situation:

1. Select AM/IMU as the first backup mode of a failed INS.

2. Note GS now is in the ballpark where it should be (in our case 0 knots)

3. Perform a FIX to get into position again.

 

At least we can do something with the aircraft now, it's not pretty but it will get us home.

 

What I tried to solve it was to go back to INS function but the READY and STBY lights remain lit, meaning the INS not functional anymore.

 

In the end, there is nothing preventing you from entering " stuff" into the Computer and in this case, it was nothing I could do (to my knowledge) that would've solved this situation.

 

My two main questions are;

A. Why does the INS calculates a groundspeed when own aircraft heading is set to, in this case, 62 degrees?

B Why didn't the system switch over to a backup mode as the GS and TAS units it got were vastly different?

 

Edited September 19, 2019 by Looney

[Pikey]

Interested in the answers to this as well. This was a big headscratcher, and with difficult to read pits in VR you can for sure have a typo, with more than just degraded INS, it's quite a dramatic effect!

 

 

From pilots perspective:

 

 

Your Pitch ladder (if enabled) looks drunk and starts to lean off

 

 

The AHRS needle remained centred, switching to other sources didnt change, pressing the reset button didnt change anything. Tid repeater obviously you wont know you are off unless you remain on the ground and the RIO happened to be watching it hooked and you are paying attention.

Edited September 19, 2019 by Pikey

[mattag08]

Kinda weird behavior since entering aircraft heading is really only useful for a HS INS align. With the AHRS working you would think the INS would just calculate and sync the heading as always even if you entered a value there at any point.

[Super Grover]

First of all, the AHRS can't be considered as a reliable and precise enough source of magnetic heading, not mentioning true heading which for which the performance is even worse. That's why the "MV" acronym appears on the TID only after the difference between the calculated magnetic variation, and the entered magnetic variation is greater than 5 degrees.

 

 

Entering own heading is useful when performing an in-flight reset of the IMU. The procedure is to:

1. Set NAV MODE to OFF.

2. Press PRGM RESTRT (not simulated yed).

 

3. Set NAV MODE to IMU/AM.

4. Fly straight and level for 3 to 5 minutes.

5. Enter own true HDG.

6. Enter winds.

7. Enter own LAT and LONG (from wingman, carrier, AWACS).

The INS mode would not be available, but it should allow using the IMU/AM.

 

 

 

 

Answering Looney's questions.

A. Why does the INS calculates a groundspeed when own aircraft heading is set to, in this case, 62 degrees?

I may have an idea of the reason for such a high ground speed drift, but it's difficult to say it for sure without seeing the debug data. However, I can try explaining it.

Earth rotates at a rate of 15° per hour, and Earth surface (with aircraft parked on it) moves at ~600 kt (for latitude ~45°). The INS must adjust for such a movement, and accounts for it in the algorithms and applies necessary corrections to the IMU platform. When you input a wrong heading, the INS applies wrong corrections, and the IMU is not levelled correctly any longer. In consequence, and the INS starts sensing gravity as a false horizontal acceleration, and it starts drifting very quickly.

With own heading error of 62°, the INS mixes East-West with North-South so heavily, that it can easily result in false ground speed readings at the level of Earth surface linear speed.

 

 

B. Why didn't the system switch over to a backup mode as the GS and TAS units it got were vastly different?

The INS itself has limited means of detecting a failure like this. GS and TAS can be (and usually are) different because of wind. And wind can be as strong as 100kt (this is the wind speed at FL300 over the Great Lakes from today). Of course, 600kt is much less probable :D, but we have to remember that it is a system from the early '70s, and it required a bit more attention from the operator when compared with modern INS.

[QuiGon]

Very interesting stuff, thanks Super Grover! I really enjoy threads like this :)

[mattag08]

First of all, the AHRS can't be considered as a reliable and precise enough source of magnetic heading, not mentioning true heading which for which the performance is even worse.

Is it not true that the system is comparing the calculated/sensed heading to the detected magnetic heading and syncing the two during periods where the aircraft is stable?

[Super Grover]

In the INS mode, the two headings - the true heading from the INS/IMU and the magnetic heading from the AHRS - are completely independent and never synced. They are used together to calculate the magnetic variation value.

[Looney]

Thanks for the explanation Super Grover, do you want me to send a file for debugging purposes or are you fine with my explanation?

[mattag08]

In the INS mode, the two headings - the true heading from the INS/IMU and the magnetic heading from the AHRS - are completely independent and never synced. They are used together to calculate the magnetic variation value.

Interesting, I must've misread or read into what the manual stated.

[WelshZeCorgi]

@super Grover Is there no way to correct this mistake once the heading is accidently inputted?