Non-Precision Approaches

I don’t know about you, but you might say that I have an approach/avoidance conflict with some non-precision approaches, in particular with VOR approaches. On the one hand, I like them because they cut me some slack if I’m feeling mellow, the weather isn’t too crummy, and the ceilings are still comfortably in the neighborhood of the transition point from three to four digits. Then again, particularly with the VOR approaches for which the navigation facility isn’t on the field, the errors can really accumulate if your navigation equipment isn’t as precise as it once was, and you don’t fly them accurately. When the chips are down (along with the ceilings and visibility), that might just leave you wondering-particularly when you emerge from the clouds and the runway (or even the airport environment) is nowhere in sight.

It isn’t an irrational fear, either; this is due simply to geometry. The further one ventures from a known point, when you’re depending upon the lateral accuracy of a signal that is resolved based on radial measure, the greater the error; i.e., the larger the circle, the longer is the chord subtended by each degree. Of course, we’ve got a newer crop of more precise non-precision approaches now, so things aren’t as bad as one might think. There are many GPS approaches, RNAV GPS approaches, and also LOC (localizer) approaches, and these do provide better lateral guidance than a lone VHF navigational facility way off in the next county. (The VOR for one approach into my home airport for example is almost 20 statute miles away from our runway.) As a matter of fact I did an informal survey of all of the non-precision instrument approaches in Maryland, my home state. Not counting the bigger airports like Baltimore or Salisbury (which because of their size would almost always have a VOR or NDB on the airport, anyway) there were just five for which the source of navigational guidance was right on the airport (four of those being the lowly NDB approach). When I added up the numbers of approaches for which the associated navigational aid was elsewhere though (such as VOR, VOR-A, NDB, and also ‘VOR or GPS’ approaches), the off-airport total was…16. (Incidentally for the cases where more than one VOR or VOR-A approach served the same airport, I counted them as just one.) How far were these from the runways they served? The closest one was less than two miles away; the furthest was almost 30 miles away. The average distance was about 12 nautical miles. When I considered my own airport, and that my VOR receiver might theoretically be off by six degrees and still be legal-I’m assuming the worst case where its accuracy was measured at the airborne checkpoint over one end of our runway-that could mean a lateral error of about two miles. Guess what? The minimum visibility for this approach is half that.

There are other fears regarding certain non-precision approaches, circling approaches in particular, that involve aero-medical, psychological, and statistical considerations, rather than just geometrical ones. It is common practice for many corporate operators to forbid night circling approaches, or to shun non-precision approaches altogether, for example. Along with the fact that a well flown non-precision approach can end with a short and possibly uncomfortable scud run followed by a fast drop onto a slippery runway in an unfamiliar area, there is also the tremendous temptation to take just a small peek below the MDA, followed by another just a little lower, and this can become significantly more likely when a pilot has flown a familiar approach many times into the same airport. I’m not naming any names, but trust me, I know!

There are also a few other things about these types of instrument approaches which aren’t inherently hazardous, but which can work against you if you aren’t making the most of your opportunities-or the information available. The first one that comes to mind has to do with the descent to the MDA or minimum descent altitude (which is the lowest altitude to which one is authorized to descend, prior to being able to land visually, expressed in feet above mean sea level). Unlike precision approaches, which since they provide vertical as well as lateral guidance deliver you more or less right at the runway’s doorstep, the traditional descent profile for a non-precision approach involves a descent from some procedure entry altitude down to the MDA, after which point the pilot usually continues flying at the MDA until reaching the missed approach point. At the bottom of the approach plates, for those VOR approaches where the navigational facility isn’t on the airport, there is a table of times to fly from the final approach fix (FAF) to the MAP, based on one’s groundspeed. More and more aircraft having GPS now provide the pilot with precise knowledge of groundspeed that is considerably more accurate than the approximations I had to deal with a decade ago, which were based on my estimated airspeed and what I had interpolated between the winds aloft at 3000 feet and the surface. But even armed with that, too slow a descent to the MDA, for fear of encountering unfriendly terrain or perhaps being publicly impaled atop some transmission tower, can leave the pilot with little time to lose those final few hundred feet and land within the first third of the runway with a ‘normal rate of descent using normal maneuvers’ as it says in Part 91 of the regulations. The trick is to get down early enough to search for the airport while remaining at safe altitudes, but at the same time minimizing your exposure to flying outside of the airport environment at excessively low elevations. (It is highly stressful, as well as patently unwise, to be still descending and looking for the airport at the same time. Even arriving at the MDA right when you reach the MAP can be nerve-racking, because that basically leaves you with little or no time in which to make your decision as to whether or not you can land.) Generally, arrival at the MDA somewhere between a half-mile and a mile before the airport usually works out fairly well.

Part of being able to do that with minimal distraction involves what I referred to as opportunity rather than information, and one of the best ways to increase your chances of completing an approach successfully is to simply reduce your workload by knowing what configuration and power settings will give you the rate of descent that you need. When I first started my instrument training in a Cessna 172, my instructor did me a huge favor by offering me the target rpm setting for a 700 foot per minute descent, which is just a ballpark number, but it’s a good deal easier to fine tune above or below a target than taking a shot in the dark (which is where I’d have been, had he not told me that). When you think about that, it makes sense. After all, we often use target airspeeds for best rate of climb, as well as airspeeds and configuration settings (i.e., flaps, carburetor heat, etc.) for the base and final approach legs in the pattern, right? Just be careful not to exceed anything over 1000 feet per minute in the descent. At those rates, it is too easy to inadvertently descend below the MDA. And speaking of distractions, as with any operation done in instrument conditions, don’t exceed your task saturation bandwidth. Granted, it isn’t easy to maintain an ever-present awareness of just how much you can handle at one time, but it isn’t terribly profound to advise that you not look down to read the approach plate while you’re making a turn, or that you enter a new radio frequency one digit at a time as you continue your scan, etc.

One rather major point that I should address is to qualify (or perhaps apologize for) my opening implication that non-precision approaches are a lackadaisical affair. On principle of course, one should fly non-precision approaches with every bit as much precision as one would fly an ILS. Then again, speaking of precision, on the other hand it is also a very smart idea to add a conservative cushion to how low (or for how long) you will fly on any approach, until you are comfortable with flying it ‘to minimums’. There are too many opportunities for errors to stack up against you, such as an error in your altimeter or variations in the local altimeter settings between where you are, and where the measurement is being taken.

There is an additional item of advice I will offer, although some may consider it a transgression from the level of procedure to one merely of technique. I was taught to use the throttle to control altitude during the descent, rather than attitude. (The expression ‘pitch for airspeed, power for altitude’ will be familiar to some.) The reason is that pulling the nose up to regain altitude results in lost airspeed and shortly afterwards, an increase in descent rate (and the opposite effects occur after lowering the nose to hasten the descent). This has worked exceedingly well for me, particularly when following the glideslope on an ILS.

Finally, here’s a last take-home point: Never shoot any approach without having a firm plan in mind as to just what you will do, along with just what you’ll need to do, if you have to ‘go missed’. (It’s a nice idea to inform Approach of this too, if you think of it.) That removes the pressure of having to extemporaneously manage strategic and logistical decisions at the same time (i.e., planning what you’ll do next, only while you’re also cleaning up the airplane and climbing away from the runway). Always brief the full approach, including the missed, beforehand. In fact, it’s safe to say that you should always plan on going missed, and being pleasantly surprised when you don’t have to.