Part IV concludes this brief overview of the AN/ASN-92 Inertial Navigation System. This part covers the means of fixing its most common issues, and discusses some of them with two former F-14 Radar Intercept Officers.
The most common issue of the INS is the drift: due to intrinsic limitations or external factors, the AN/ANS-92 may “think” the aircraft is in a position not matching the real location. Usually, updating the “Fix” solves the problem.
Sometimes, however, the problem is more complex and requires more effort to sort it out.
Parenthesis: INS, vC and Positional Imprecision – A Chat with F-14 Radar Intercept Officers
In my long quest of understanding the F-14 and its avionics, I stumbled upon the imprecisions caused by Carrier Operations (see Part III of this series). It is indeed one of the very first things a virtual RIO notices when operating from a Carrier the first times. I was almost shocked by the magnitude of the difference between the actual heading indication provided by displays and the “real” heading.
A spontaneous question soon arises: “how did the crew deal with this problem in real life?”
No one can answer this question better than former F-14 RIOs!
I had another conversation with Dave “Bio” Baranek. He flew the Tomcat in the glorious early days, having joined the Navy in 1979. In his book, Tomcat RIO, he talks about a TARPS mission. A picture (right after page 102) represents a flight plan, something along a PPL dead reckoning-based log, to give you an idea. However, if the INS was so imprecise, how could they thoroughly follow the scheduled plan?
Although “state of the art” when designed, the [Tomcat’s navigation systems] seemed out of date and barely adequate just a few years later. But to get right to the point: in my experience it wasn’t as bad […] Most of my F-14 flying was pre-GPS. But in my recollection, heading info was accurate unless you had a system failure — which was rare.
[…] As we flew ahead of the carrier after launch, the pilot pushed the HDG Pushbutton and that made everything line up. This was routine. In fact, forgetting to do this was the basis for a brief episode in Tomcat RIO:
(The situation was my pilot Hooter and I were discussing a navigation error during a flight)
“I forgot to synch the compass,” he said. Pilots usually did that right after takeoff as they flew away from the enormous metal mass of the carrier. Don’t ask me to explain magnetic fields and their effect on our swing-wing jet fighter’s compass, I just know that when Hooter pushed the Synch button our displays shifted by 20 degrees. We were 60 miles south of our station! The INS was fine, as things turned out.
(In the end, this instance and another led us to realize we’d become complacent.)
But that was the only time in my 2,500 hours flying Tomcats that I had this problem. Of course, it wasn’t an issue when we started up ashore. On the other hand, the INS could drift or outright fail, and I mention that several times in the book, but I actually mention the possibility of it failing, and how we took steps to be prepared for that. It didn’t actually fail that often.
In any case, heading on the BDHI was accurate to one degree unless we had a real problem […].
Dave “Bio” Baranek – F-14 RIO
I also had a chat with Scott “Weird” Altorfer, Radar Intercept Officer in the VF-154 and VF-201, from 1993 to 1998. Being a RIO in the “post-GPS era” changed many things, and provided an incredible tool to improve and simplify navigation.
In fact, it looks like I overestimated the problem!
[…] in the real world, we simply did not care that much about the levels of precision you describe, probably because everything pretty much worked most of the time, and if it didn’t, we had a wingman and a GPS.
[…] If my INS dumped mid flight, I’d break out my little book of tests, run them on the way home, and write a gripe in maintenance control.
If my compass was off up to 10 degrees, I might not even notice! Radio navaids meant a needle points to them; landmarks on the ground don’t move, and the boat was always steaming around, anyway, so finding her was both an art and a science.
[…] For my TARPS missions over Iraq, I had my route programmed into both the Garmin and the INS. If they agreed, great, if not, I’d use the GPS.
Scott “Weird” Altorfer – F-14A RIO
Both comments are fascinating. From one hand, we can assess the revolutionary impact the Global Positioning System had in the military, before it landed in our daily lives. They are also witness of the ingenuity of people that found a way to improve their life by means of a simple commercial device.
On the other, they are a testament to the technical quality of a device developed more than 60 years ago that, failures aside, was capable of precisely guide a fighter jet to their objectives.
Translating this into DCS is not immediate: different players look for different gaming experiences, some issues are not existing (random failures can be activated, but they are arguably enjoyable). Nevertheless, these conversations give us a couple of ideas for more realistic ops:
the usage of the GPS (NS 430). Often dismissed and probably not worth the full price if purchased only for the F-14 (unless a 3D model is introduced), it is definitely a nice addition in a post ’90s mission. It enables more complex and precise flight plans and map-scale navigation;
the AN/ASN-92. As discussed in the previous chapter, the re-sync of the AHRS happens automatically, but it is a factor to be aware of, and worthy of being part of everyone’s SOP. Although the question about the functionality of the HDG Pushbutton in DCS remains (I have raised it with the devs already), the Radar Intercept Officer, even the Casual player, should be aware of this problem and work with his pilot to solve it when operating from a Carrier.
I also had a chat with a former A-6E Intruder B/N (Bombardier/Navigator). To my great surprise, we realised that the INS works apparently quite differently on the two aircraft.
But this is a story for another article 🙂
Troubleshooting More Complex Issues
Rarely, other and more potentially serious issues can occur. The first step is identifying the problem.
“STANDBY” and “READY” Lights
These two lights are located above the NAV MODE knob, in the top-left corner of the Tactical Information Display (see the following picture, STBY is top and READY is bottom).
They are sometimes barely visible when the cockpit is illuminated by direct sunlight, but are fundamental to understand the status of the navigational systems.
The following table helps the Radar Intercept Officer to troubleshoot the most common issues through the two lights.
Normal display for first 45 seconds after initialization of the alignment.
A failure in IMU, NAV COMP, NPS or AHRS prevents the system from performing satisfactorily in the selected mode due.
Select another mode.
Normal status during alignment after initialization.
Normal status during erection of IMU when IMU/AM is selected before to completion of coarse align.
NAV Mode switch should be left in selected position to obtain the NAV system alignment.
NAV Mode switch should be left in selected position. Until lMU is erected, the AHRS/AM mode will be provided automatically.
STANDBY and READY Flashing
Suspended alignment prevents the initialization of the alignment.
Check brake pressure and set parking brake.
Set parking brake to continue the alignment.
The alignment satisfies the minimum requirements to enable weapons launch.
IMU/AM or INS are available with AHRS/AM selected.
NAV Mode switch should be left in the selected position for improved alignment or switch to INS.
Select available NAV Mode.
The NAV system is off or the system operating satisfactorily in the selected mode.
READY Flashing (after 5 seconds both OFF)
Status displayed only when the IMU/AM mode is selected and platform is aligned. The RIO has 5 seconds to select another NAV Mode, or alignment will be lost and the INS will not be available.
Select another mode to avoid the loss of the alignment.
With parking brake off, alignment suspended past mission alert criteria (second tick).
Set parking brake, select INS or select the desired mode of operation.
The “MV” Acronym on the TID
The MV acronym can appear on the TID for multiple reason. One is an undetected failure of the AHRS that can cause an incorrect value of MagVar to be computed. More commonly, MV appears when vM and vC differs by 5° or more. The computed MagVar and the manual MagVar are, in fact, continuously compared, and if the case above manifests, the IN acronym appears alternated with MV (note that IM may be substituted, rather than IN, depending on the selected navigation mode).
During carrier operations, this phenomenon is expected: the aircraft carrier can affect the computed magnetic variation, as discussed in Part III of this series.
The following table provides a checklist useful to identify and action the problem.
The MV acronym is displayed alternately with NAV mode in flight with no AHRS, IMU or NAVCOMP failure lights illuminated.
RIO manually enter new (and corrected) MAG AR.
MV acronym should be cleared.
The conditions in step 1 persist after the actions in step 1 are performed.
The Pilot should compare the heading on VDI HUD with the heading on VDI or HUD with standby compass while in SLAVED COMPASS mode, INS or IMU/AM NAV mode, and level unaccelerated flight.
If comparison agrees, the problem may be the IMU. Continue to step 3 to verify this.
Skip to step 5 otherwise.
The IMU is the suspected source of the wrong vC.
The Pilot switches to COMP mode in the AHRS compass controller and compares VDI and HUD headings with standby compass.
If the comparisons agrees, the IMU heading is erroneous.
IMU heading is erroneous.
The RIO selects AHRS/AM Nav Mode, and enters the correct MagVar.
MV acronym should be cleared.
The VDI/HUD headings did not initially agree with the standby compass in step 2. In this case, the AHRS is the problem source for vC error.
The Pilot synchronizes the AHRS by depressing the HDG pushbutton.
If the synchronization is not possible, switch to COMP mode.
If the Pilot has switched to COMP mode, all computer and CRT display functions will now use the true heading provided by the IMU with the manual MagVar applied.
The BDHI now receives information from the Magnetic Azimuth Detector (MAD). The output may or may not be correct, depending on what has failed in the AHRS.
The MV acronym may or may not be cleared.
Note: The two tables reported in this part of the INS series, useful to troubleshoot the INS, will be added to my Kneeboard Pack at some point in the future.
Positional Fix Update
The most common issue affecting the INS and its operation by the crew is the Drift. This problem has usually a very marginal impact, and be corrected in different ways.
The following is a list of the various methods to update the Fix discussed in the past. Each method is viable in a different situation, for example, the proximity of TACAN stations.
Generally speaking, each consist in the determination of a “delta” between the aircraft position assumed by the INS, and the reference location or a specific waypoint. The Radar Intercept Officer can evaluate such delta and take action.
TACAN update: this method evaluates the difference between a waypoint and a TACAN station;
Visual update: a waypoint is created over a recognisable landmark, and the F-14 flies over such position;
Radar update: often complex to use, a specific ground feature identifiable via the DDD is compared to a specific waypoint;
Others: definitely non-realistic approaches are available, such as using the NS 430 to update the position of the aircraft;