Pelican’s Perch #24:
Sloppy, Sorry VNAV

Flying a non-precision approach has traditionally been a


Pelican's PerchI suspect Cap’n Jepp is rolling in hisgrave over the latest Briefing Bulletin – Vertical Navigation (VNAV), dated 12 NOV 99 -from Jeppesen in Denver. Not that it’s Jeppesen’s fault; they’re just charting the latestindustry foolishness. But Captain E. B. Jeppesen was an airman of the old school, and Ibelieve he would have rejected VNAV descents during non-precision approaches in all but avery few cases.

This time, it’s not the FAA, although I don’t exactly see them doing any good in thearea, and they’re buying into some pretty bad procedures. As Pogo said, “We have metthe enemy, and he is us!”

VNAV, in this case, refers to the deplorable tendency to make wimpy gradual descentsduring the final descent to MDA (Minimum Descent Altitude) during non-precisionapproaches, when an immediate step-down is better, followed by a period of level flight atMDA to the MAP (Missed Approach Point). Some pilots will even work out the math inadvance, and preplan a rate of descent that will put them at the MDA at the MAP at aboutthe same time! I’ve had people do this on check rides, and when I fussed at them for notgetting down sooner, they look puzzled and say “But I got down at the MAP!”Yeah, but barely! I don’t fail them for it, but it will draw a comment from me in thedebrief. You’re supposed to get down in good time to make a normal landing on the runway,and it doesn’t matter where the MAP is!

Many pilots seem to think this glide-slope-like descent is a classier or easier way todo this maneuver. Maybe it is easier, but it often doesn’t get the job done, it’s not theway the procedure was designed, and above all, it can be dangerous.

With the advent of glass cockpits, inertial systems, RNAV, approach-approved GPS, andfancy computers, this technique is even being institutionalized and computerized, with”pseudo-glide slopes” being constructed and put right in the databases of thesesophisticated systems. And now, alas, Jeppesen is formalizing it with the data right onthe approach plates, presumably for everyone to use!

Yes, yes, I know, it says, “Use of this descent angle by certified VNAV-capableavionics equipment will ensure a stable, constant rate of descent that will clear allintervening altitude restrictions.” I suspect that one-liner will get lost in theshuffle, and we’ll see a lot more people trying to use the technique without adequateequipment. Further, even with adequate equipment, we’ll see a lot more missed approaches.Meanwhile those who know what they’re doing, and who do it in the classic manner, willquietly, legally, and safely make the approach and landing while the glass cockpits gomerrily to the alternate.

Bad idea, I say … BAD idea.

Back to Basics

Bear with me, please, this may be a little different way of looking at this subjectthan you’ve seen before. First we need a couple of definitions, and a mini-review of thebasics.

There are two basic types of SIAPs (common shorthand for “Standard InstrumentApproach Procedure”), the “Precision,” and the “Non-Precision.”

  • The precision approach has vertical guidance for the descent.
  • The non-precision approach does not.

Pretty simple, eh? (No, I’m not Canadian.)

While those are the basic definitions, “precision” also implies highlyaccurate horizontal and vertical guidance to, or very near, the landing area of therunway. Most common is the sixty-year-old ILS system, which provides a LOC (Localizer),and GS (Glide Slope), plus other components. There are a surprising number of other types,PAR, MLS, TLS, WAAS, LAAS, GLS, AND SCAT-1, but none are germane to this column.

“Nonprecision” approaches are “all those others,” with no verticalguidance. Examples of non-precision approaches are Localizer-only, VOR, ADF, Loran, RNAV,current GPS, etc.

All the rules for constructing these approaches are contained in FAA Order 8260.3B”United States Standard For Terminal Instrument Procedures (TERPS).” It is veryimportant stuff for those designing SIAPs, but bedtime reading material for the rest ofus. It’s heavy going, with lots of angles, formulas, and very rarely any explanation ofjust WHY the designers chose to do something a certain way. I have to move my lips whenI’m trying to puzzle something out from it.

If you’ve read even a little of TERPS, it should have become crystal clear that theprocedures shown on those approach plates should be followed, and followed very, verycarefully, lest the ground rise up and smite thee. The Rockwell Number of granite is muchhigher than that of aluminum – or skin and bones!

There are two further approach variants beyond “precision” and”non-precision.” One is the “Straight-In,” and the other is the”Circling” approach. The latter is not well understood by many general aviationpilots and for that matter, some airline pilots who fly only into airports served by anILS. In fact, most airlines now flatly prohibit circling approaches entirely, unlessthey’re done in basic VFR conditions. If they can’t land straight in with lower weatherconditions (preferably using ILS), they go somewhere else. I suspect ole Cap’n Jepp wouldshake his head at that, too, for the death of circling approaches represents the loss of auseful tool, and it’s another loss in overall airmanship.

In fairness, a low circle with a jet transport is not a trivial matter, and unless theairline is prepared to spend a lot of money on periodic training on this specificmaneuver, the risk is probably not worth the reward. Until recent years, the visualdisplays in simulators were simply not capable of showing the view during a low circlewith any accuracy, and mostly as a result of this, they are generally banned for jetstoday. Ironically, the very latest simulators do a credible job at showing the low circlein real time, but I guess no one has the heart to bring them back. Too bad, I love them -at least in the simulator. A low circle in minimum visibility with one engine out, and a50-knot direct crosswind will get the old adrenaline a’pumpin’, for sure!

VORs, NDBs, and Other Strange Critters

But for the purposes of this column, let’s narrow the subject down to straight-innon-precision approaches, primarily VOR and NDB, for these are the ones the glass-cockpitfolks are a’monkeyin’ with.

There are two separate and distinct requirements for a straight-in, non-precisionapproach to be established:

  • The final approach course must lie within 30 degrees of the extended runway centerline.
  • Whatever final course is chosen, it must comply with the obstacle clearance criteria in TERPS.

To give you an idea of obstacle clearance requirements, here is a generic pictureextracted from TERPS, cleaned up, and some color added. There are many variations on this,but this will give you a general idea.

TERPS Approach (3-view)

To give you some idea of how much variation there is, here’s the specific diagram forthe VOR approach:

VOR Approach Segments (with FAF)
VOR Approach Protected Areas

There are a couple of interesting things about this diagram. Note the faint innerdotted line. While it is shown in all three segments, it really has significance only forthe final segment, and the extension of those dotted lines form the boundaries of thatfinal segment. The area within that dotted line can be up to 30 nm long! Its only purposeis to make the final segment wider if the VOR is further from the runway.

Many pilots seem stuck on the idea that a non-precision approach is to the runway, andsome of the TERPS requirements seem to give that impression. It may even be technicallycorrect. But in reality, a non-precision approach in a non-glass cockpit is pretty looselyattached to a runway, and in most cases, you’ll be much better off thinking of it assimply an approach to the general area of the runway, or even to the airport itself. Hereis perhaps the simplest possible alignment of the final approach course, from a VORlocated away from the airport:

VOR Approach (facility off airport)

Note this diagram has nothing to do with obstacle clearance! This simply showsthe limits for the angles of the straight-in, off-airport VOR. The perfect final approachcourse must fall within the blue area, and that area can lie anywhere between the30-degree lines from the runway. In rare cases, it might even be a nice straight shot!

Now let’s take a look at the on-airport VOR case:

VOR Approach (facility on airport)

At Last, a Point!

Yes, I’m finally coming to it.

Now, look at the blue areas depicted on the last two graphics. Those tell the designerof the VOR approach where the precise CENTERLINE of the approach path can be. The MAP mustalso be within the blue areas.

But that’s perfection, theory only!

TERPS also allows for errors in the ground facility, errors in the airborne radios, andsome considerable sloppiness of the pilots, too (well, for me, at least). This is wherethe obstacle clearances enter the picture, and there are a whole bunch of graphics andtext describing those. All you need to understand here is that those final approach pathsare ideals, and the reality can be very different.

By way of example, at the legal maximum, a VOR located 30 nm away, the protectedprimary area at the runway will be FIVE miles wide, or two and one-half miles each side ofthe exact centerline.

Maximum permissible error of your airborne VOR is four degrees, give or take, right?You are supposed to know what the actual error in your equipment is, and allow for it, butall things considered, four degrees of total error (ground radios, too) is not anunreasonable thing to think about. Four degrees of error at 30 nm is about two milesoff-course, barely within the TERPS criteria for protected airspace! Remember that lurkinggranite, just beyond the obstacle clearance boundary.

For a Localizer approach, the glass cockpit folks can punch in their data, hit the FAFor the “pseudo-glide slope,” and slide on down just like it was a real glideslope. The VNAV path is computed to the landing threshold, and the navigation is veryaccurate, being continuously updated by double-DME fixes, or GPS, or the like. I agree,that’s nice.

Where that CANPA (Continuous Angle Non-Precision Approach, a new term) intersects theMDA, a VDP (Visual Descent Point) is established.

The other non-precision approaches are done the same way with glass cockpits, butunlike the Localizer, there is the added complexity of the final approach probably notbeing aligned with the runway centerline.

If that VDP on the VOR or NDB approach just happens to align pretty closely with therunway, then it too becomes a good deal with the glass cockpit magic, because you’ll breakout in a good position to just keep on descending to the runway without changing a thing.Once again, the extreme accuracy inherent in the glass cockpit assures that you will beright on the approach path plotted by the designer, at a perfect altitude to just continuewith a nice, stabilized descent, but possibly with up to a 30-degree turn to the runway(new criteria may limit this to 15 degrees for these approaches). I’m still not fullyhappy with this, because there are a lot of times you can do it the old way, break out ina clear spot, then maneuver just a little bit to stay clear of clouds (sometimes called”Dive and Drive”). Doing it with the magic will cause more missed approaches,but perhaps it is worth it, in terms of requiring less airmanship and skill, which makesit safer.

Why is the glass cockpit so magical, and so much more accurate than the older”steam-driven” cockpits as found in 727s, older 737s, and all 747s except the-400?

The key is that the glass cockpit is NOT using the VOR for the VOR approach at all!Yes, you read that right. The folks on the ground could shut the VOR station down right inthe middle of the approach, and it would not only not affect the approach, but the folksin that glass cockpit probably wouldn’t even know it!

How does this happen? These cockpits are usually using inertial reference systems, withautomatic updating from any available source. It’s a pretty smart system, it will usuallypick the most accurate possible method of updating itself, and it’s doing it all the time,as long as any radio facility is available. If GPS is hooked into the system, that’llprobably be the primary source for updating the inertial units. Lacking GPS, it willprobably auto-tune two DME facilities as close as possible to 90 degrees apart from theairplane, do the math, and slew (update) the inertial systems as needed to match. Theresult is that the nav computers in the airplane know exactly where the airplane is, downto a few feet with GPS updating (or GPS as the primary), a few hundred feet with doubleDME. If there are no outside radios available, the inertial systems are capable of greataccuracy all by themselves. We still go across the Pacific Ocean on the inertial unitsalone, updating only when “coasting in.” (Most airlines have not adopted GPS,yet.)

Now, if you tell this “magic” to “go directly to XYZ VOR,” it doesNOT tune in XYZ, and go there. It mumbles to itself, takes a quick peek into the databaseand looks up the latitude, longitude, and elevation of the XYZ VOR, does a quickcalculation, and says to itself, “If I’m at this lat/lon, and XYZ is at that lat/lon,the course from ‘here’ to ‘there’ is 123 degrees.”

It gets better. It “remembers” where “here” was, and where”XYZ” is, and does whatever is necessary to get the airplane on the Great Circlebetween those two points, keep in on that course with great accuracy, and headed in theright direction. Whether XYZ VOR is working or not simply doesn’t matter, it’s probablynever even used, or tuned by the system! When you program the magic for the approach, ithits XYZ dead on, available or not. After XYZ, it not only does all the real-timenumber-crunching to keep the airplane on the Great Circle to the MAP, but it also computesthe proper descent angle to clear all crossing restrictions to arrive at the runway end.All this time, it’s also driving a nice big color display in the cockpit with a movingmap, and a glide slope needle, keeping those humans informed.

(“What’s it doing now?” “I dunno, but it’s doing it with fantasticaccuracy!”)

You may wonder “Well, why not just go ahead and tune that VOR to be safer?”Go ahead, try it. You’ll be appalled at the wild needle swings, and the common errors inthe VOR system! You’ll sometimes see that VOR needle swing full-scale, while the magiccalmly flies a dead-straight course, never wavering, never seeming to correct. If I’mdestroying your faith in VOR, GOOD! VOR navigation is slap-dash at best, and always hasbeen (though still better than ADF). Take THAT a step further, and recall the last timeyou shot a VOR approach, with high terrain all around!

Now perhaps you can see why the magic will do such a great non-precision approach. Itwill reliably place the airplane exactly on the precise centerline the approach designerselected.

If you use the magic to do the non-precision approach to minimums using this”Constant Angle Non Precision Approach (CANPA)” with a ceiling at minimums, youwill have only a few seconds to look for the airport, because now the MDA altitude actsjust like a DH/DA, it becomes a DECISION POINT. You might well pop out of a minimumceiling, not see the airport, and have to immediately execute the missed approach, becauseby definition, you’ve popped out at the VDP, and if you go past that, you won’t be able toland safely even if you do spot the runway.

Without the Magic, Back to the Basics

If you use that shaky old VOR to get there while trying to make a gradual descent ofsome sort, and even if you time it right to hit MDA right at the VDP, your position whenyou pop out of the ceiling could be anywhere at all, well outside those blue areas! You’llstill be safe from the terrain, because the design of the approach (probably) gives yousufficient room for error, but “Where’s the airport, Charlie?” It may be left,it may be right, it may be underneath where you can’t see it. You probably won’t have thetime to find it visually, and even if you do see it, it may take a near-acrobatic maneuverto get to the end of the runway, while completing the checklist, getting final flaps out,etc. Unless you just happen to get lucky, all you can do is miss the approach, try again,or go away. Yes, it’s real easy to sit in a comfortable chair behind a desk, as chiefpilots do, and say “Well, you shoulda done the miss!” The reality is that at theend of a long day, maybe the final landing at home with a hot date waiting, you just maymake that split-second decision to try it. “Yeah, it looks a little worse than I’dlike, but let’s give it a try, we can always do a go-around, right?” Try thisin the rain, at night, with a black hole and a bit of turbulence, and you’ve got a recipefor disaster.

Without the magic, the old way is better. That means you descend early to MDA, leveloff there beneath the ceiling (or even still in the clouds), and start looking forfamiliar landmarks, or signs of that elusive runway. How early do you start down? Well,remember that once you cross the last fix before the MDA leg, you could, if you had ahelicopter, descend vertically just past that fix to the MDA. Few of us want to carry itto that extreme, but for very good reasons, it’s more than appropriate to do it the wayCaptain Jeppesen did in the early days of instrument approaches, descend as rapidly aspossible to MDA, and level out there. For practical purposes, 1,000 feet per minute afterthe fix is a good target to use, although some higher speed jets will need to use a bitmore to even get down before the VDP or MAP, on some approaches.

Once at MDA, maintain at or above it (for check rides, it’s +50, -0 feet) until theairport is in sight and it is time to descend, or until the MAP, then miss. Normally,descent to the MDA will put you beneath the ceiling, giving you a good deal of time tolook for the runway long before the glass cockpit folks would have even popped out of theclouds. You will have an early opportunity to make any needed turns for lining up with therunway, and you will probably need that maneuver room, considering the inherentinaccuracies in the non-glass, non-precision approach. Your only job will be finding therunway, then descending on a reasonable final approach to touchdown.

All that said, I don’t care how you get to that final fix before letting down to theMDA. You can step it down, or you can fake some sort of easy descent path, but I dosuggest you hit that fix AT the minimum altitude, and fully configured for a fairly rapiddescent. As soon as that fix is passed, DUMP IT, and hustle right on down to the MDA justas quickly as you can get there safely (“Dive and Drive.”) You may need to putout gear and full flaps, and reduce power to a very low setting, but whatever it takes,short of Herculean efforts, GET DOWN to that MDA! As you approach the MDA, you’ll want toease the power up, and perhaps pull the flaps back up to some reasonable approach setting.On the prop airplanes, it’s best to get back to a configuration you’d use with an enginefailure, so you don’t have the cleanup problems if one does fail. By doing the approach inthis manner, you should catch sight of the runway very early, and will have plenty of timeto get the flaps (and possibly the gear) back down again. Of course, if the airplane willfly well with an engine failure and the gear down, you probably ought to leave the geardown when you hit MDA, so there’s one less thing to do.

Sometimes the old ways are still the best!

NOTE: I am indebted to Wally Roberts, the world-class expert on TERPS, who has taught me so much but not enough. He kindly skimmed this column for gross errors. He gave me the new buzzword “CANPA,” and suggests it’s a good thing for jet transports. I agree with that for the glass cockpit magic, but not for the “steam-driven” jets with the old electronics, many of which are still flying. For the average GA airplane, the old way is by far the best.

Be careful up there!