On Instruments: Diving and driving

Most pilots prefer the safety and lower landing minimums provided by the vertical guidance of an Instrument Landing System (ILS). Even so, nonprecision approaches can come into play at larger airports when adverse winds affecting a runway served by an ILS approach dictate flying a nonprecision approach to another runway. And for those of us with a need to fly to more remote airports with shorter, less-well-equipped runways, nonprecision approaches often rule.

Do nonprecision approaches pose special challenges? You bet they do, and if mis-handled a procedure can bite back. The biggest challenge has to do with the approach’s higher landing minimums. Typically, a nonprecision approach will have a minimum descent altitude (MDA) of 500 feet or so above the runway threshold elevation, and a visibility minimum of one statute mile. You can’t legally fly below those minimums unless you have the runway in sight and can complete the landing. Approaches with vertical guidance typically have minimums of 200 feet and one-half mile—and following a precision approach’s localizer and glideslope needles helps give assurance that you’ll be in a proper position for a straight-in landing.

Dive and drive

A nonprecision approach begins like most other approaches: by navigating from an initial or intermediate approach fix to the final approach fix (FAF), or ATC providing vectors to intercept one of the final legs of the approach procedure. There may even be a procedure turn to reverse course on the extended final approach course, or a trip around a holding pattern in lieu of a procedure turn. It all depends on the published procedure. Many older VOR approaches (where the VOR is located on the airport) don’t even have FAFs. Instead, a procedure turn on the final approach course radial outbound often serves this function.

Passing over the final approach fix, inbound on the final approach course, things get busy—fast. In quick succession you must configure the airplane for the upcoming landing by reducing power, extending the flaps and landing gear, trimming the airplane for the approach airspeed and, if necessary, starting your approach timer. Meanwhile, you also begin your descent to the MDA.

And what a descent it could be. Again, it varies, but often you’ll need to lose at least 1,000 feet—and maybe even 2,000 feet or more—of altitude on your way to the MDA. The traditional advice is to begin the descent to the MDA immediately after passing the final approach fix, and descend aggressively—at, say, 1,000 fpm or more, rather than a leisurely 500 fpm. The idea here is to descend quickly through any clouds and into better visibilities early in the approach. Once at the MDA, it’s time to add power, level off, and retrim as you motor along the rest of the final approach course. This leaves you with more time to spot the runway and get ready for the landing. This is the well-known “dive and drive” procedure that most of us have been taught.

Well before the missed approach point (MAP)—which can be defined by distance, fixes, navaids, or elapsed time from the FAF—you’ll hopefully see the runway environment in enough time to land out of a reasonably stabilized approach profile. But let’s say the visibility is at minimums and you suddenly spot the runway. You’ll have to make a steep descent to arrive over the threshold, and do it without gaining airspeed. The chances of that are slim. Instead of flying at 90 knots and descending at 400 fpm, you could find yourself flying down short final doing something like 110 knots in a 1,000-fpm descent.

(Images in slideshow are reduced for illustrative purposes only. Reproduced with permission of Jeppesen Sanderson, Inc. NOT FOR NAVIGATIONAL USE. © Jeppesen Sanderson, Inc [2020])

Does a descent rate like this sound too risky? Some pilots might find this too challenging, for good reason. There’s not much time to judge your distance and prepare for the flare. Plus, a lot of variables are at work. Fly faster, have a tailwind component on final or a higher MDA, and final approach descent rates go up—along with landing distances.

Where’s the MAP?

Nonprecision missed approach points can be defined in several ways. They can be at a runway threshold, or a VOR located on the airport. With RNAV GPS approaches, these and other MAPs can be defined by their published distances from the FAF, the same way DME information is used during VOR-DME approaches. VOR bearing cross-fixes are another option. Elapsed time is the trickiest, because it’s based on groundspeed; you’ll have to adjust your time based on the wind’s effects.

It bears emphasizing that you must stay at the MDA until you see the runway and are in a position to make a “normal” landing. Don’t see the runway? Then fly to the MAP and execute the missed approach.

The danger comes when, in the heat of the moment during an approach at minimums, you see the threshold too late. Even though you can make out the runway, you’ll be at the MAP in seconds. The big temptation is to try to rescue the approach by diving at the runway. But this sets you up for excessive floating in the flare; landing long; a runway overshoot; a hard landing; or worse. It’s safer to fly the missed approach procedure.

VDPs

There have been improvements to make some straight-in nonprecision approaches safer. Some of these approaches publish visual descent points (VDPs) using a V-symbol along their final approach courses. They’re usually defined by published RNAV GPS or VOR-DME distances and can be located anywhere from 1.5 to 10 or more nautical miles from the runway threshold.

At a VDP, visual descents from the MDA to the runway landing zone can be made using normal descent profiles, and precise visual descent angles to the threshold are also published. Using a VDP is a sort of insurance that does away with the need for any last-minute slam-dunk descents and makes the final approach more stable—like those of precision approaches. Of course, to use a VDP you need to be in visual conditions, and must be able to complete the approach visually.

VDPs look even better when you understand that approaches without VDPs haven’t been examined for terrain clearance below the MDA—so there’s no assurance of a clear path to the runway at normal descent angles.

Continuous descent on final approach

Continuous descents on final approach provide a single descent angle for flying directly from an FAF to a VDP. This lets you avoid the “dive and drive” and promotes stabilized approaches. The descent angle is published, along with a dotted line indicating the approach path. Continuous descents on final approach are published on Jeppesen approach plates and are relatively new and, while they’re not required of Part 91 pilots, the FAA and NTSB encourage their use.

LNAV/VNAV and LPV minimums

These satellite-based GPS approaches, leveraging WAAS (Wide Area Augmentation System) technology, can provide vertical guidance, but strictly speaking they’re not precision approaches. The FAA calls them approaches with vertical guidance, or APVs for short.

Yes, these offer glideslope-like vertical guidance and generally lower minimums than nonprecision approaches. But their signal characteristics and availability don’t match ILS standards. LPVs often have DAs with values equal to 200 or 250 feet above a runway threshold’s elevation.

Because they also permit gradual descents favoring stabilized configurations, these approaches are great alternatives to nonprecision approaches lacking vertical guidance, such as pure VOR, VOR-DME, some localizer/LDA, and even—gasp!—NDB approaches. So if LNAV/VNAV or LPV minimums are published and your airplane is properly equipped, I’d use them. Anything to avoid descending through step-down altitudes or fixes that culminate with a slam-dunk on short final.

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