An Aviation Safety Staff Report
Weve discussed the differences and conflicts between various types of approaches before but, if there is any procedure that meets the good news/bad news test, its the generic non-precision approach, or NPA. On one hand, we like them because they cut us some slack if were feeling particularly mellow, the visibility isnt too low and the ceilings are still comfortably high. Thats the good news.
The bad news is that, well, an NPA is less crisp than its big brother, the ILS. There are many reasons for this-some of which well explore-but for now understand that the bad news isnt restricted to the lack of an electronic glide path. When the chips are down-along with the ceilings and visibility-theres no ILS around and very little about your arrival leaves you mellow, you need to know how to get the most from what you have.
1. Geometry
One of the first things we need to consider when shooting an NPA is the type of electronic guidance well be receiving and where the navaid is located. Of course, for a GPS procedure, this isnt a factor. But for VOR- or NDB-based approaches, the transmitters distance from the runway will, in part, determine how low we can go and the lateral accuracy we can expect when looking for the airport.
This is due simply to geometry: The further one ventures from a known point, when depending on the accuracy of an analog signal, the greater the error. For example, the VOR for an approach into one of our editors home airport is almost 20 sm from the runway. An informal survey of all of the non-precision approaches in Maryland reveals that, not counting the larger fields like Baltimore or Salisbury, there were just five for which the principal navaid was right on the airport. And four of those were NDBs.
The problem with all this is that, when we consider that a VOR receiver may theoretically be out of specification by up to six degrees and still be legal, the math could translate into a lateral error of about two miles.Guess what? The minimum visibility for many NPAs is half that. So, we need to consider the navaids proximity to the runway when executing an NPA.Basically, the further one is from the navaid on the final approach segment, the more centered we need to keep the needle.
2. Circling
Weve also talked about the circle-to-land maneuver after a successful NPA.Because of the need to maneuver the airplane for landing on the appropriate runway, a well-flown NPA can end with what amounts to a short and possibly uncomfortable scud run followed by a fast drop onto a slippery, unfamiliar runway. The circle-to-land maneuever, in our opinion, is a valuable and necessary tool but, just like any other, one must know how to use it.
Circling is essentially a visual manuever with IFR rules. It can be thought of as a low-altitude traffic pattern where, by definition, the margin for error is less. One must simultaneously fly the circle-to-land maneuver at a constant altitude-until a normal descent can take you to the runway-at or above the published circling MDA but below the cloud layer. You must turn and bank within a defined radius of the airport and keep the intended runway in sight. This means a busy cockpit, and more than one crew has failed to execute this balancing act.
Even though circling is FAA approved, regularly taught and incorporated into the Practical Test Standards for the Instrument rating, it is commonly prohibited by scheduled carriers and more than a few corporate operators. If they cant use an ILS or land straight-in after an NPA, they aint going. In some quarters, their reluctance might serve as a wake-up call, but most of us flivver drivers are used to banking and descending relatively close to the ground. After all, we fly VFR patterns and even if we get a little behind the power curve, our airplanes are light and responsive enough to recover. Just remember that arresting a high descent rate on the final approach course before turning to circle also requires adding a lot of power, especially with the gear out.
3. Timing
Unlike precision procedures, the traditional non-precision approach involves a descent from some altitude down to the MDA, after which the pilot usually continues flying at the MDA until reaching the missed approach point (MAP).At the bottom of the approach plate, for those NPAs where the navaid isnt at the missed approach point or which dont require GPS or DME, there is a table listing the time required to fly from the final approach fix (FAF) to the MAP, based on ones groundspeed. With GPS and a moving map, its a snap to know your distance to the MAP-just following the little airplane. Without it, youll still need to do the timing.
As well expand on in a moment, one of the keys to properly timing an NPA is knowing what power settings to use in your airplane. Once established on an intermediate segment outside of the FAF, the airplane should be in level flight and mostly configured for the approach. Whether you drop the landing gear at the FAF or-for example, if your gear is welded down-reduce power to initiate the descent, the idea is to fly the airplane at a constant airspeed, using power to control pitch and thereby altitude. This makes the timing exercise simpler and much more consistent, improving your chances to get onto that runway the first time.
Without a direct groundspeed readout in the panel, subtract the estimated surface wind from your true airspeed-thats the groundspeed value to use when timing. Just before reaching the MDA, smoothly add back the power you pulled off to descend and you should be in level flight, at the MDA, and ready to find the runway.
4. Descending
One of the best ways to increase your chances of completing an NPA successfully is to simply reduce your workload. Knowing what configuration and power settings will give you which rate of descent can be invaluable when shooting an NPA. If you and your CFII havent gone out and determined which airplane configurations to use for, say, 400-, 600- and 1000-fpm descents on NPAs, pick up the phone right now and schedule some dual for that purpose.
After all, we often use target airspeeds for best rate of climb, as well as airspeeds and configuration settings (i.e., flaps, gear, carburetor heat, etc.) for the base and final approach legs in the pattern, right? Just be careful not to exceed anything over 1000 fpm in the descent. At those rates, its too easy to inadvertently descend below the MDA.
The trick is to get down early enough to search for the airport while remaining at or above the MDA 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. Generally, arriving at the MDA somewhere between a half-mile and a mile before the MAP usually works out fairly well.
In recent years, the visual descent point, or VDP, has been added to NPAs. The VDP is the point at which you can reasonably expect to see the runway from the MDA and start down with a standard three-degree descent angle to the runway. Do not confuse the VDP with the MAP. If you want to create a VDP for a procedure lacking it, use the published height above touchdown value at the MDA and divide it by 300. This gives you the distance from the runway threshold at which you can begin a normal descent to the runway. Presuming, of course, that you have the runway in sight.
5. Missing
Finally, here is one 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 youll need to do, if you have to go missed. Its a nice idea to inform ATC of what your plans are, too. For example, will you want to come around for another attempt at the same approach, request a different procedure using a different navaid or using a different runway, or divert to another airport, perhaps one with an ILS?
Having some kind of plan in mind before leaving the FAF and descending helps keep you ahead of the airplane and removes the pressure of having to make strategic and logistical decisions on the fly while youre also cleaning up the airplane and climbing away from the runway. Always brief the full approach, including the miss, beforehand, even if youre the only one receiving the briefing.
With any approach, its better to plan on having to execute the missed approach procedure-including navigating to the holding fix and (gasp!) actually entering the hold-and being pleasantly surprised when you dont have to.
One final thing: Dont forget that the ILS youre looking to shoot can quickly become a non-precision, localizer-only procedure, even after you leave the FAF. Glideslope antennas have been known to suffer damage, aircraft have taxied beyond the ILS hold line and more than one airborne receiver has failed at an inopportune time. Even if you think youre executing a precision approach, you might be left without that electronic glideslope somewhere between the outer marker and the threshold. If you plan ahead to revert to the localizer-only procedure, you wont be the least bit surprised when the glideslope flag rears its ugly head.
Also With This Article
“Defining The Non-Precision Approach”
“CANPA? Or Dive And Drive?”
“When To Start The Miss”
