I was tickled that the HUD article (head-up display, not Paul Newman) Joe Sweeney and I wrote for the winter 1986-87 issue of Code One created more than a moderate level of interest. However, on several occasions, in several places, I have been stung by expert opinions about how the HUD in the F-16 is fraught with insidious failures. Holders of these opinions appear to have more than their fair share of metaphysical insight, extrasensory perception, clairvoyance, intuition, crystal vision, metapsychosis, or a priori knowledge. I suspect what they don’t have is a general knowledge of just what the HUD is all about. Or, worse yet, they don’t have the objectivity and independence of thought not to parrot some staff officer’s attempt to lend credibility to the edict from on high: HUDs will not be used as a primary instrument reference.
To be fair, though, not all of the article’s readers were so confused. Many are honestly trying, as I am, to contribute ways to improve the fighter pilot’s knowledge of the HUD. They will not be put on my list of dilettantes. (That’s dilly taunts, for you Ag majors.)
Because this is important, I have done some exhaustive research on the subject and have put together a list of insidious failures with the HUD in the F- 16. Here they are:
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Well? Do you notice a pattern starting to emerge?
See, it’s not that hard to break the code. There are none. Passionate declarations about all these failures just lend another data point to the Drydenian theorem of conservation of intelligence, to wit: For every PhD, there is always an equal and opposite PhD. Furthermore, since the head-down attitude direction indicator, or ADI gets its attitude information from the same source as the HUD, that is, from the inertial navigation system, or INS, why wouldn’t these same insidious failures be just as insidious with the ADI? My experts don’t want to answer that one.
So I’ve made my point. Let’s take a few minutes now to expand the discussion on the use of the HUD that Joe Sweeney and I started a few issues back. Please recall the original article we offered as Part 5 of the Semper Viper series in Code One. Everything still applies. Joe made the very valid point that the HUD is no better or worse than the sources from which it derives its information. For example, the form, fit, and function INS that is provided on the F- 16 is government-furnished equipment, or GFE, and is the source of information for the pitch ladder and velocity vector. Even more important, Joe mentioned the built-in self-test in both the source black boxes and the HUD itself. The HUD is designed with very stringent requirements that result in the fact that, if anything is the least bit suspicious, the HUD no longer displays that part of the information. In addition, the systems will report the detected failure on the pilot’s fault list, or PFL, so there is no doubt that the failure took place. Thus, the pilot can make allowances for the problem.
How will some of these failures look to you in the cockpit? For instance, what if you have an INS dump completely? What kind of clues can you expect? The approach that Joe and I mentioned still applies. If the HUD doesn’t like the information it is getting, it doesn’t display it. Period. Therefore, the velocity vector will disappear and the pitch ladder will probably freeze or disappear completely. It is apparent even to the most casual observer that you have a problem. Your corrective action? If you are on visual flight rules, no sweat. If you are in the soup, go immediately to the standby ADI, as the normal head-down ADI gets its information from the same source as the HUD (and also will most likely be wrong). Only after you are sure that the ADI head-down is working should you stop using the standby as the center of your crosscheck. After you are sure you are upright, you can check the PFL and I’ll bet it says the INS failed. Nothing insidious here.
But what if you have a problem with the central air data computer, the CADC, or the air data converter, the ADC? An ADC 003 that indicates a problem with the bus traffic from the ADC to the HUD will result in a PFL to that effect. However, you should know well ahead of the point that you consult the PFL as the airspeed and the altitude scale numbers disappeared when the malfunction occurred. The tic marks on the scales still remain as an indication that you have the scales selected, but all the numbers have vanished. Nothing insidious here, either.
In my earlier observations about the HUD, I mentioned the only possible insidious failure – and even that one required an obvious failure plus a switch change on your part. That one involved the erroneous pitch information: ok straight and level. But as you roll in some bank, the pitch indication is wrong and gets worse as the bank angle increases. This can happen only if there is a fire control computer, or FCC, failure after you have selected drift cutout. (You must have both present; neither by itself can produce the same result.)
Should this occur, you will know immediately. In the C/D aircraft, you will get a PFL displayed on the data entry display. But in the A/B, it will be displayed only on the face of the fire control navigation panel. In both cases, nearly all the digital info on the bottom of the HUD will disappear. If you are looking at the radar, you will notice that nearly all the target info has disappeared. If the gear is down (in either model), the angle-of-attack bracket on the HUD will disappear. I don’t think this can be classified as insidious, either. We learned that this was the result of an equation screw-up in the HUD, and it is not present in Block 40 aircraft and is being corrected in the Block 25/30 airplanes. The corrected equations have been supplied to Ogden for the Block 15S birds. In the meantime, if you detect an FCC failure, make sure you’re not in drift cutout. So let’s face it. Possibly one, and only one, insidious failure. And that is being rectified.
Now having said all that, my years of experience still tell me that there are no absolutes. The engineers assure me that the system is designed to work a particular way, and I feel that is the case. I am not saying that some off-the-wall, totally insidious failure is not possible. (My innate suspicious nature warns me against that). I’m a pilot first and an engineer second. So keep your wits about you. It just might happen that the groundspeed calculation in the INS runs away and gives you some screwy indications without tripping the maintenance fault list, or MFL, flags. So use your head. If the display changes some weird way or if it is not responding to a control input you are attempting, be wary. You are the pilot. The government does not pay you a bonus over the ground-pounders just because it thinks you are a good guy or you are particularly good-looking. Earn the flight pay you take home.
In the Dash Thirty-Four, you will notice several PFLs that state that the HUD is degraded. Most of these have to do with deflection voltages that mean some of the information will not be presented in exactly the right place on the HUD display. For example, the pitch ladder will not necessarily be exactly to the correct scale. But the velocity vector will still be displayed with the correct relationship to the pitch ladder so the aircraft control is not in question. Before I get any I-told-you-so’s from the nitpickers, may I point out that is what you have all the time with the ADI The thirty- to forty-plus degrees in both directions you can see on the face of the instrument is not real world, either. Nevertheless, the PFLs are important if you are participating in Gunsmoke, Long Rifle, and so forth, as it does have an effect on the aiming references on the HUD. These PFLs and MFLs are certainly part of ensuring that the F-16 is completely operational for the competition. Or for combat. Write them up when they happen.
My conclusion, as you may have guessed by now, is that trying to find reasons why the HUD cannot be used is futile. All of those you can find are bogus. Let’s get on with the program.
Aside from the dilly taunts from the uninformed about alleged dastardly, sneaky failures of the HUD, I have been asked some really good questions that I want to address. I just had to get this other stuff out of the way. (Thanks for hanging in with me so far.)
The first involves using the HUD for an instrument takeoff. It can be done very nicely. This point, and those that follow, have to do with just what information you are looking at as you face the HUD. I talked about the velocity vector in the previous article and how this is the different piece of information that is missing from any ADI you have used in the past. But the HUD still has all the information that any ADI had before, that is, all the pitch and roll information plus all the performance instruments conveniently grouped close by. (Okay, okay. No tach. Not that important.) The velocity vector provides the information as to just what the flight path of the airplane is doing and you can determine what this is by viewing it directly. This is what you had to do indirectly in the past by observing and interpreting the ADI and the several performance instruments that go along with it. Since all the information is still present on the HUD that was included on the entire instrument panels of the past, you can use the HUD in the same way you did in your previous airplane. I mentioned using the standby reticle before, but the same holds true with any fixed part of the HUD.
So if you cannot see the velocity vector for any reason, you can still fly the aircraft head-up using the HUD. During an instrument takeoff while the aircraft is still on the ground, for instance, the velocity vector is useless since the aircraft is not yet flying. If the INS has failed and you have done an in-flight alignment, the velocity vector will not be available. If you are at an angle of attack greater than about fifteen degrees, you will not be able to see the velocity vector off the bottom of the HUD. If you are very slow with a lot of crosswind, the velocity vector will be off one side of the combining glass. In every case, the HUD is still perfectly useful. Take the time to determine the approximate size (in degrees) of all (or at least most) of the fixed displays on the HUD. Once you have a feel for their size, you can then make pitch corrections against the pitch ladder in exactly the same manner you do with the ADI approach in any other situation. In other words, any fixed part of the HUD is exactly the same as the small aircraft symbol you have been using on the ADI in the past. The only difference is that the small aircraft happens to be in the center of the instrument.
Consider an instrument takeoff. As the F-16 sits slightly nose-down on the ground, you will notice that the horizon line is just above the top of the airspeed and altitude scales. Notice also that the ten-degree down-line is just below the info at the bottom of the HUD display. So it stands to reason that the display is close to ten degrees wide, n’est-ce pas? So during a takeoff where the visibility is truly poor, just add backpressure until the horizon line has moved down to just below the info at the bottom of the HUD display. In doing so, you will have raised the nose of the aircraft very close to ten degrees: right in the middle of the eight to twelve degrees the Dash One calls for. Wonderful.
You will see the velocity vector start to move up, indicating the aircraft is now off the ground and climbing. See how easy that is? From that point, continue using the velocity vector as you normally would. The same holds true any time the velocity vector is not displayed on the HUD, but you still have a valid pitch ladder; that is, you have not received information to the contrary through the warning and caution system. Even if you can see only the very end of the pitch ladder lines, you can still make one, two, or nearly any number of degree inputs by referencing the pitch lines to any fixed part of the HUD display and pushing or pulling on the stick.
I am not saying that this is any kind of ideal presentation. But I am saying that there is absolutely no reason to become disoriented or, worse yet, lose control of the airplane just because you have a less-than-perfect display for that short a period of time. So, even if I were faced with a partial INS problem that did not allow use of the velocity vector, I would try to obtain an in-flight alignment, turn on the standby reticle, turn on the vertical velocity scale (I don’t use it if the velocity vector is working), and proceed to fly using the HUD the same way I used the ADI and so on in previous airplanes. What I just described doesn’t really exist. But with a subtle software change, the HUD could look to the CADC for the vertical velocity instead of just turning it off when it detects bad INS poop. This certainly should be a change candidate. Think about it. This technique is much easier since all the information is in the same place; hence, an easier crosscheck. Only if I couldn’t obtain a valid pitch ladder on the HUD would I resort to using the head-down displays. In other words, there is a lot of information on the HUD that is there for the taking if you choose to make use of it. A good example of that is the point I made in the previous article about the ability to use the C/D HUD to determine the radial I am presently flying on or through.
Simply perform the following mental gymnastics. Look at the heading of your jet, take the relative bearing of the bearing pointer that you have selected with the latitude/ longitude of the TACAN station, and add to or subtract from your heading. (For example, if the relative bearing is to the left of the nose, subtract. If the needle is to the right, add the relative bearing to your heading.) Then take the reciprocal and that is the TACAN radial you are on. Sounds a bit difficult, but, with a little practice, it will become a snap. To give a practical for-instance, I have the coordinates of the TACAN station at Nellis AFB under a convenient waypoint. I am heading 180 degrees (south) and the relative bearing to the waypoint is ninety degrees left (pointing to the left wing tip). I have to be within a degree of the 270-degree radial out of the TACAN station. A big boost to my situation awareness if I am in the weather. It appears that it will be possible to install the C/D wide-angle conventional, or WAC, HUD in the A/B airplanes during or after the operational capabilities upgrade. The WAC HUD is going in the A/B airplanes we will be delivering to the latest A/B customers. The present HUD in the A/B is perfectly adequate and better than any existing head-down ADI system. But the WAC HUD is still nicer.
You C/D drivers will like the subtle changes that the 30B software will provide in the HUD. The horizon line has been extended to provide a better clue (a further extension would be better still) and a roll index has been added at the bottom of the display. Although some of you guys will probably like this a lot, I have mixed emotions on this one. I would like to see the roll index as a declutter option since I still get all the roll info I need from the position of the velocity vector in relation to the pitch ladder. I would be interested in hearing your impression when you get a chance to see it.
Therefore, to make short work of it, the insidious failures that the Chicken Littles keep talking about do not exist. First, the HUD is designed to make it plain, in no uncertain terms, that it has a failure. And failures truly are few and far between. I personally have had only four problems with the HUD in thirteen years of flying the F-16. They were very apparent when they happened.
Second, the HUD provides a wealth of information that contributes greatly to the pilots’ SA as to the attitude, energy state, and positional information that is there for the asking. Why not use it? Better, easier, and more precise control of the aircraft will be the only result.
