Pelican’s Perch #25:
How I Learned to Love CANPA

John Deakin's

Pelican's PerchNo, I haven’t changed my mind about theusefulness (and the dangers) of the classic early descent to MDA and flying along levellooking for the runway, nor the usefulness (and the dangers) of doing CANPA, the so-called”Constant Angle Non Precision Approach” when appropriate. But my previous column obviously needs a little follow-up. Like theseven blind men, each feeling an appendage of the elephant, everyone seemed to have adifferent perspective of what I tried to say. So I’ll say it again, in a different way,and probably make em all even madder.

(Yes, I know, it’s four blind men. I prefer my version.)

There are a wide variety of airplanes, auto-flight systems, pilots and approaches”out there,” and what works for one combination is deadly for another. It’s avery complex subject. My first column did not emphasize this point, and a lot of nicepeople jumped on me as a result. But keep those cards and letters coming, folks, I loveem!

I really am very pleased at the avalanche of reader mail I got. The column also spawneda very long and healthy “thread” over in CompuServe’s AVSIG Forum. Regardless of”right” or “wrong,” I think many people learned a lot, and most ofall, gave the subject some much-needed thought. A columnist can ask for no more.

Some Terminology

My friend, mentor, and sometime-tormenter Randy Sohn takes violent objection to thecommon shorthand of “Dive and Drive” for the old-style NPA (Non-PrecisionApproach). He feels it has a terribly negative connotation, for it is not a”dive,” it is a controlled, planned rate of descent to the final MDA, with onlyminimal configuration changes at the start and end. He and I agree that it’s very simpleto configure before that descent, perhaps note the power setting, make a simple powerchange (often to a known value), do the descent, and then reset the previous power at orapproaching MDA. Or just do whatever it takes to put the airplane where you want it, butthat seems to be a dying skill.

Unfortunately for Randy and other purists, “Dive and Drive” is a part of theaviation lexicon, and like it or not, it’s a common shorthand that differentiates onemethod from another. For some pilots, it’s a pejorative word, for others an affectionateshorthand. But out of deference to someone vastly older than I am, I shall avoid the useof “Dive and Drive,” and see how “Classic NPA” works.

To further define terms, I use “power” here generically, intending to include”thrust.” I do love to irritate the purists who get all bent out of shape overthe differences between “throttles,” “thrust levers,” and “powerlevers.” It drove me nuts when my company insisted that in the DC-8 they were”throttles,” and in the 747 and 727, “thrust levers,” and thenactually got uptight when pilots would slip and say one instead of the other! In rockets,according to NASA, it’s “throttle up” and “throttle down,” so gofigure. All I know is “push to go, pull to slow,” which works fine in all theairplanes I fly, prop or jet. Maybe we need a new term, to cover em all. We’ll alsoneed some sort of variation for the French, who don’t even want the levers to move, nomatter what you call them.

Pinball Wizards

737-600panel.jpg (115367 bytes)
737-600panel.jpg (115367 bytes)
737-600panel.jpg (115367 bytes)

Those who screamed the loudest at my “blast” at CANPA seemed not to be thereal pilots at all, but people like Airbus drivers who are no more than airborne videogame players, disassociated from all reality, at least until the ground rises up andsmites them with reality – and finality. For them, if it can’t be done with the push of abutton or three, they aren’t interested. Worse, they assume the whole world must do it thesame way, and that anything else is unsafe.

Why, one fellow I know, condemned to flying the French Fry … er, flying pinballmachine, says he only flies it on the auto-flight system so it won’t screw up his realflying skills on real airplanes. Smart man.

When one of these button-pusher-drivers is requested to do a sidestep approach, hewhines about re-programming the stupid computer! Heck, during my usual hand-flown ILS, mynormal excursions probably overlap the centerline of the other runway, so all I have to dois roll wings level at that point, and I’m all set for the sidestep, and a new set ofexcursions! No problem.

As another side note, didja ever notice airline pilots will scream bloody murder about”safety” unless they have a personal reason for doing something, or there’s moremoney involved? It’s really funny how that works. If the captain has a hot blonde waiting,he’ll ASK for the sidestep to save a second or three.

As we all know, the real measure of skill for an Airbus pilot is being able to type 60words per minute on the world’s worst word processor. Check your flying skills at thejetway, pal, they’re not needed. In fact, they are a detriment. Did you know that when aperson is type-rated in any of the Airbii, his pilot certificate is amended to read:


Hey, would I lie?

No, I won’t repeat the hoary old joke about the dog, everyone’s heard that one by now.

There! All that ought to draw a little fire! Gosh, that felt good!

Seriously, let’s do a few real scenarios, starting with the Airbus-type equipment. No,I’m not qualified in any Airbus, and don’t want to be. I like flying and computers, but Iprefer to have the computers help me, rather than me helping them. Ooops, there I goagain, Airbus drivers (never pilots) are such an easy target, and they scream so loudlywhen wounded.

While I’m at it, let’s lump all the 757/767/777 “drivers” in there withem. 747-400, too. They’re all a buncha auto-flight freaks, if you ask me.

(In case you hadn’t noticed, I thought I’d have a little fun with this column.)

Magic Airplanes


Boeing 737-600
Boeing 737-600

Boeing 777
Boeing 777

Using the Airbus as one example among many, I think we can all agree that there are anumber of airplanes flying today that are capable of fixing their position in space withgreat accuracy, automatically, all the time. (That’s great, I love it – the airplaneknows where it is, even if the crew doesn’t.) These airplanes usually have some sort ofredundant inertial system, updated with constant GPS inputs or double DME inputs. This isall automatic, transparent to the crew – they don’t even have to tune the radios. Thesystem knows where the best stations are and which afford the best geometry, and theyauto-tune them without pilot action. All these aircraft have varying degrees of”computing power” that neatly ties in where the airplane is and where the crewwants it to go. In some cases, the accuracy is an astonishing few dozens of feet, or less.

The ideal system is probably inertial updated by GPS. While the inertial system aloneis capable of maintaining great accuracy for many minutes (we cross the Pacific oninertial alone), it will eventually drift off by a few miles. Not critical for oceanicflying, but not good enough for terminal operations without updating. With GPS or DMEconstantly “nudging” it back towards perfection, you can bet your life on it,and we do. Second best is probably inertial updated by double DME. Some airlines and someFAA people prefer the double DME, not trusting GPS quite yet. Eventually, GPS alone maybecome reliable enough to be used completely stand-alone for all aircraft, but “notyet.”

Magic airplane, full ILS

Now, if we take one of these flying videogames and put it on a full working ILSapproach, it uses all that “magic” to find the localizer, then the glide slope,where it transitions to the older-style “auto-coupled” approach, often includingthe landing and rollout. A point worth noting here is that the localizer and glide slopesignals converge, and thus get much more sensitive as the aircraft approaches, and muchmore accurate. The terrain under that slope has been surveyed with a high degree ofaccuracy, and is protected in many ways by law.

Airplanes of the future will probably all do this, automatically, all the time, withouteven having the ground transmitters for the localizer and glide slope. I make a lot of funof it, and some denigrating comments about “drivers” vs. “pilots,” butit may even be a better way, I don’t know. What I do know is that to me it’s not”flying,” and I’m glad I served as a pilot during the period I did, and not thisnew one. I’ve never flown a HUD (Head-Up Display), but I think I’d like it a lot better. Istill do all ILSs hand-flown, unless the weather is so low that only an auto-coupledapproach is legal. So I’m a dinosaur – what else is new?

The above is not the so-called CANPA approach; I mention it only as a startingpoint, a model to which all other approaches are compared.

Magic Airplane, LOC approach

Now for the next scenario, let’s take that same situation, same magic airplane, butwith the ground transmitter for the glide slope inoperative. The flying video game will dothe same thing, but this time instead of following an external electronic path (a radiatedglide slope from the ground), it will use its internal accuracy, its knowledge of whereit is at any given moment, its knowledge of the position and altitude of the beginning ofthe final descent, and its knowledge of the point in space 50 feet above the threshold ofthe runway to compute its own electronic glide slope in the cockpit. Some systems don’teven need the final descent fix, you simply tell them what degree slope you want, and theycompute when to start down. The auto-flight system (or even a mere human) can follow that”glide slope needle” just like a real one. Fly level to the descent point andstart down, following the needle the system gives you a nice, steady slope right to therunway, just like an ILS glide slope. Good system, I like it – on a LOC approach. Again,it is worth noting that this “pseudo glide slope” does not get moresensitive as the airplane nears the landing! On the very close final, the old ILS is stillthe most accurate system in common use. For this reason and others, even with the”magic,” a localizer-only approach is limited to slightly higher minima than thefull ILS, because it is still a “Non-Precision Approach.” One day, if we everget one of the long-planned enhancements to GPS (like WAAS or LAAS), we may be making trueprecision approaches and even auto-landings with it, but that day is not here, yet, forline pilots.

The full ILS is a true “Constant Angle” approach and also a precisionapproach. The LOC only example above is a “Constant Angle Non-PrecisionApproach,” or “CANPA.” The airplane will follow the exact same path inspace, regardless of wind, drift angle, or speed as long as the needles are centered. Ifthere’s a tailwind, the groundspeed will be higher, and the auto-flight system willincrease the rate of descent to track the glide slope, whether the autopilot is flying, orwhether the human is just following the needles. If there’s a strong headwind, the systemwill automatically decrease the rate of descent, again maintaining that fixed slope inspace. Like the full underlying ILS, the terrain is known, and terrain clearance isassured.

I have no problem with that, in fact I rather like it. From a general safetystandpoint, it makes the LOC approach just like the ILS (for the “magic”airplane), albeit with slightly higher minima.

Magic Airplane, VOR (or NDB) approach

Be careful, now, this is going to get complicated. Let’s take the same airplane, same”magic,” and a VOR approach where the final is aligned with the landing runway.Under those very specific conditions, you can turn off the VOR ground station entirely asfar as I’m concerned, the “magic” is far more accurate than the VOR system. (Theprudent pilot will keep it on and visible as a backup, of course.) You can tell the”magic” the latitude, longitude and elevation of the point to start the finaldescent (or let some systems compute it themselves), and the latitude, longitude andelevation of the point 50 feet above the runway threshold, and have at it. Go ahead, makeit a CANPA, at least to MDA. But you’d better be aware that from MDA down to the runway,you may NOT have the same terrain clearance you’d have on an ILS system. Unless, ofcourse, it’s been specifically surveyed (as it would have been for an ILS), you have noguarantees at all. In fact, there are situations where making a CANPA all the way to therunway will put you below obstructions on the way from MDA to the runway!

What’s that? I can hear someone saying “So what, I’m gonna miss at MDA if I don’tsee the runway, and if I do see the runway, I’ll see any obstructions.”

Uh, huh. Suuuure you will. Tell me, which is easier to see when the rain is streamingacross the windshield: the end of a brightly-lit runway or one of those tall TV towers?

You will probably miss more of these approaches by doing the CANPA than if you did it”the old way” (for reasons explained in my previouscolumn), but the increased safety from not letting Airbus drivers do something unusualmay well be “better” than getting in those few extra times. That’s an economicdecision, and not in my job description.

See how complex this gets?

Magic Airplane, Offset VOR (or NDB)

Next step. Take the same “magic airplane” and a VOR (or NDB) approach that isNOT aligned closely to the runway. Remember, a so-called straight-in NPA can be upto 30 degrees off the runway centerline, and does not even have to ever cross it. Yes,there is a movement afoot to limit CANPA approaches to 15 degrees off the runway, but”not yet.” Again, the aircraft systems are capable of delivering you to a pointin space much more accurately than the underlying system, and I like the idea of using the”lateral nav” functions to do that. If you do a CANPA on this type of approach,descend only to MDA, then do an immediate miss when you don’t see the runway, you’re safeenough, but you’re not doing the job most of us are paid to do. If you do the CANPA andsee the runway, you are going to be instantly faced with a whole bunch of variables,things to do, and decisions to make. You are going to have to align with the runway,perhaps making a small correction to the glidepath. You are going to have to keep in mindthat you may not have obstruction clearance. And finally, you’ll have the usual battlegetting all the magic disconnected, when it really doesn’t want to go away.

If faced with that last approach (30 degrees off the runway) in an airplane with allthe “magic,” I’m pretty sure I’d prefer the “classic” early descent toMDA. All on autopilot, of course – God forbid one of these “drivers” shouldhave to fly.

(Didja hear about the Airbus that got stuck in a holding pattern for three solid hours,crew couldn’t break it out? They had to get a phone patch to Airbus, find someone whospoke English, and get tech support to help them out? Toughest part of it was getting pastthe “All of our technical support people are busy helping other customers, your callwill be answered in the order received if you speak French, otherwise forget it.”)

Non-Magic Airplanes

Boeing 747-200
Boeing 747-200


Boeing 707
Boeing 707

But let’s quit picking on the Airbus drivers, and switch now to the airplane I fly, a”Classic” 747 (also known as “Rope Start,” “Steam Driven,”and other pet names). Triple Inertial Nav, triple-channel autopilot, triple ILS/GS. We’llarrive at the terminal area with the inertial systems showing anywhere up to a few mileserror, for we do not have automatic updating, and we need the VORs for the arrival. We cantune in a couple VORs, or DMEs, and update the INSs about an hour out, but once we startthe descent, we’re switching the nav radios from one station to another too quickly to usethem for updating. So essentially, the INS is giving us attitude and compass stabilization(slaving), maybe a rough distance to the next fix or the airport, and that’s about it.

In the terminal area, we will navigate in the old manner, from VOR to VOR, or moreoften, use radar vectors to the ILS approach. We rarely use anything else, because weserve only the major international airports. We’ll somehow get on the final approach,follow the localizer, then the glide slope just like the Airbus did above, either onauto-flight, or hand-flown. No problem.

No Magic, LOC Only

Now turn off the ground transmitter for the glide slope again.

Without a glide slope being radiated, and without the “magic,” there isabsolutely no way we can generate an accurate glide slope, either for display, or for use,either by the auto-flight, or if flying by hand. Nothing, nada, zip. (This will be analien concept to Airbii pilots.)

Some operators, perhaps having heard about the “magic” and CANPA, have hadwhat they think is really bright idea. They’ll do the trigonometry, and figure “Well,if we hit this point here, and our ground speed is x, and we maintain exactly 763 feet perminute descent rate, we’ll have a CANPA, too!” Jepp even charts this stuff on manyapproaches.

Well … no, you won’t have CANPA. Let me point out a couple wee factors, here.

First, you’d better have available a good, accurate TAS (True Air Speed), or you’dbetter be willing (and able) to calculate one. How many of you still have an E6B, or aJepp confuser that will even do TAS, for use when the TAS indicator has fizzled? There area LOT of cockpits without such a backup, and I’ve heard there may be one or two withoutTAS indicators!.

Next, you really need a pretty good idea of the wind that will affect that finaldescent, and if the wind at the FAF is different from the wind at MDA and touchdown,you’re going to need to do some pretty good guessing on the fly to come up with a goodaverage. That’s really fun when the wind changes direction, as it usually does.

How accurate is your vertical speed indicator? Ever put a stopwatch on it, and trackedthe errors you’ll find from airplane to airplane? BIG question, how accurate is theauto-flight vertical speed selector? On the ones I fly, set 1,000 fpm, and you might get1,500, you might get 500.

Given a point in space, say 7.2 DME from the airport, have you ever watched how manyvariations there are in pilot technique? Some will cheat a few tenths early, some a fewtenths late, some will roll that vertical speed selector to -2,000 fpm for a”brisk” nose over until the actual VSI shows 1,000, then they’ll match it, somewill wimp it down a few hundred feet at a time. All fine on a “classic NPA”where there are all kinds of errors factored in, but all will have an effect on a”simulated CANPA.”

How much effect? Remember, all those errors can be accumulative. But let’s takea mere 10-knot total error on a 140-knot final. A five-mile final is 30,000 feet long. 140knots is 233 fps (feet per second), 150 knots is 250 fps. At 140 knots, it’ll take 129seconds to fly that final, at 150 knots, 120 seconds, for a nine-second difference. Bigdeal, you say. Nine seconds at 150 knots is 2,250 along track, and on a 3-degree glideslope, that’s 117 feet above or below the glide slope. For only a ten-knot error, and noother variations that I listed above.

Suppose your actual vertical speed misses the target by only 100 feet per minute.That’s not much error. Maybe you can fly a vertical speed within that tolerance, but Ican’t. In two minutes, you’ll be 200 feet high, or 200 feet low.

Not a big deal? Go try it, and see how bad it looks if you’re 100 feet high or low on areal GS, at DH or MDA! A real glide slope puts you within a foot or two of where you’reaccustomed to being, 100 feet high is downright scary in a big jet!

Of course, the folks who love CANPA love simulators, where I’m guessing the procedurewas developed. We don’t have real pilots running simulators anymore; they are often just”simulator instructors” with very limited real-world experience. The simulatoruses an utterly precise wind model, known in advance, the VSI selector gives exactly 700fpm when you set 700 fpm, and it all just works beautifully. Folks, I don’t give a rat’sposterior how well it works in the box, it doesn’t work all that well out on the line, inreal weather, even assuming perfect technique! Yes, you may have done it for real. Youmight do it several times for real, and see it work out. But let a few of those errors allaffect things in the same direction, and you’re not going to be very happy with where youwind up, either high or low when you break out and see the runway.

You just might have been better off to do the “classic NPA.”

No Magic, VOR/NDB Approach

Now it gets really interesting. Remember, with no “magic,” there are errorsin the signal radiated from the VOR, there are additional errors in the aircraftreceivers, and terrain, ice and weather can have additional effects, not to mention alittle sloppy flying. No, of course not, you never fly sloppy, but I sure do. Auto-flightsystems don’t even track VORs very well, and none that I know of will track on the ADF atall (remember, no “magic” here). Try that CANPA approach on these, and you’ll betotally unable to predict where you’re going to break out, high, low, left, right, early,or late. If you break out at the last moment, you will have a whole bunch of things to puttogether, and it can be a real scramble, probably not a good idea in a big jet.

Oh, and remember, there’s that TV tower that is sticking up into the slope you’rehoping to maintain to the runway when you can see it, and it’s raining, rough, and dark,with your alternate going down the tubes. Or perhaps this is your alternate, and it wouldbe really, really nice to get in, the first time? Yeah, right, I know, youralternates never go down the tubes, and you’ve always got a nice, above-minimumsalternate. Riiiiiight.

A Real Non-Precision Approach

Nope, for my money, if you’re going to do NPAs and you don’t have the”magic,” you’re simply better off, more likely to make it in, and probably saferdoing it the classic way. Hit that final fix in the minimum-flap landing configuration,checklists complete, and note the approximate power (thrust) setting. Start a positivedescent somewhere between 1,000 and 1,500 fpm, just before reaching MDA reset the previouslevel-flight power, and hold MDA or slightly above, while glancing outside, looking forthe runway. Use any reasonable means to get a better shot at the runway, like “Mr.Brown’s Barn” if you know the approach that well (if you don’t, fly it as charted).Once visual, as you see the proper glide slope developing (from below), start easing itdown to intercept it, all the while watching for that TV tower.

If you can’t do that, then don’t do them at all.

Bonanzas and NPAs

Most of the same comments apply, except small airplanes are far more maneuverable, andcan tolerate being high or low, left or right, far more readily than the big jets. In myBonanza, on a 10,000 foot runway, I can be over the end at 500 feet, and still landsafely. You can’t do that with a big jet.

Remember, the NPA was designed from the outset not to put you in a landing position,but to simply deliver you to the general area of the airport in weather suitable formaneuvering to land. Some are better than others at doing that, and some will even deliveryou pretty close to the end of the runway. But you can’t make ice cream out of equinewaste, and you can’t make a precision approach out of a non-precision approach.

On a Personal Note …

It is Christmas Eve as I finish this column, past deadline, as usual. If the powers at AVwebactually consent to posting this, the 25th Pelican’s Perch, it should appear on or aboutJanuary 3, 2000. Assuming most of you are Y2K compliant and have made it through thatoverblown barrier, please allow me to wish you all the very best in the coming year. I’mvery fortunate, have perfect health, few problems, far too many things to do, and aboveall, some wonderful friends, many of whom I’ve never met in person. I refer to you, myreaders, as well as others. You know who you are.

Be careful up there!