Visual descent points are handy tools the FAA has included on many GPS approaches to keep you from flying dangerously unstabilized approaches and to prevent pilots from hitting all sorts of obstacles. But there are a number of approaches that don’t have these magic little references. Read on to find out how to compute your own VDPs in one simple formula.

What Are VDPs?

In last Monday’s article we took a look at the requirement for a “normal rate of descent using normal maneuvers” to proceed below the MDA per the FAR/AIM. Unfortunately, there are a lot of missed approach points out there that have the smell of a trap. There is simply no way to make the runway using any semblance of a normal maneuver.

A Visual Descent Point (VDP) symbol

To limit the temptation to proceed with a landing under unstabilized approach conditions and prevent collisions with obstacles along the final approach path, the FAA began publishing visual descent points (VDPs) on many GPS plates. Marked by a “V,” the VDP is the last point at which a descent from the MDA to to the touchdown zone can be made at a stable three degree glideslope. If the runway is not in sight by the VPD, a missed approach should be executed.

Unfortunately, VDP’s are not published on all charts. Luckily, there is an easy technique to compute your own.

Making Your Own VDP

First, figure out the height above terrain (HAT) of the MDA. Now, divide the HAT by 300. The number you get is the distance from the runway threshold (in nautical miles) of your visual descent point. In mathematese:

VDP = HAT / 300

Let’s take an example. Consider the localizer approach to runway 2R in Nashville. Note that the HAT at the MDA is 550 feet. To make the mental math easy (aren’t we busy enough up there?) let’s round it up to 600 feet.

Recall that VDP = HAT / 300, so we have to compute:

600 / 300 = 2

The VDP for this approach is 2 miles from the runway threshold. But wait, there’s more. How will you know when you are precisely 2 miles from the threshold?

Notice that the runway threshold is at a DME of 1.5 from the localizer. Just add 1.5 + 2 to get our DME reading of 3.5 at the visual descent point.

Familiarizing yourself with a new airport? Put that Airport / Facility Directory away and fire up YouTube! Is that sectional chart a little too ambiguous? Fold that chart up and “fly” the route with Google Earth. See how modern websites are changing the way tech-savvy pilots plan their flights. In his article, Vincent from PlasticPilot.net evaluates several online tools for flight planning and preparation.

A word of caution: As great as many of these resources are, pilots should stick to official sources as the primary means of flight planning.

How often do you shoot an instrument approach? Chances are that it is not very often. Most of us have the good sense to stay out of the weather when conditions are marginal. Furthermore, in most parts of the world, the weather is usually conducive to a visual approach. This is good news for VFR pilots, but it can make the instrument rated aviator more than a bit rusty.

Airline pilots do this sort of thing on nearly every flight. When the ATIS indicates a visual approach is in use, flight crews typically brief that “this will be a visual, backed up by the ILS.” This means that we will plan on making a visual approach, while preparing for the instrument approach. This dual visual/instrument approach has several advantages.

First, the pilot’s situational awareness is boosted by the use of radio aids. We’ve all lost sight of the runway at some time or another, but a quick check of the localizer needle can provide an at-a-glance reassurance that you haven’t blown through the final approach course! Furthermore, GPS systems can be configured to display instrument approaches (and even visual approaches in some cases) as an extended centerline miles away from the runway. So long as you can fly the airplane to that line and make the turn, you ought to find the runway right in front of the nose.

Another advantage of backing up the visual with an instrument approach is that it simultaneously sharpens both visual and instrument skills. By monitoring the CDI and glideslope indicator, the pilot is more likely to maintain the perfect site-picture for a stellar landing (so long as speed control is right on!). Furthermore, the pilot’s mind will be forced to interpret and understand localizer and/or glideslope indications, leading to greater skill and confidence when landing in actual instrument conditions.

For IFR traffic operating into busy towered airports, it is strongly recommended that pilots be prepared for the most likely instrument approach as controllers may issue clearances and speed restrictions to specific approach fixes. For example, a pilot may be “cleared for the visual 18L, maintain 140 knots to RONEE, contact tower 119.7 at RONEE.” The prepared pilot will already have configured for the ILS and is already in a position to identify RONEE without scrambling for charts in a high-workload environment.

It is important to note that backing up a visual approach with an instrument approach is not the same thing as flying an actual instrument approach. These approaches cannot be logged as instrument approaches for the sake of maintaining currency unless it is done under a training hood with a qualified safety pilot.

At all times, the pilot should keep in mind that he is flying a visual approach, which does not have a published missed approach procedure. In the event of a go-around, entering the traffic pattern would be the appropriate maneuver unless otherwise directed by air traffic control.

Flight instructors across the nation have been involved in a massive coverup scheme surrounding VOR stations. We’ve all been taught to think of VOR stations as a giant compass rose transmitting radio beams in 360 directions. It is common to refer to a VOR as a giant wheel with 360 spokes representing radials. These are all lies. 

It’s the age-old “black-box” argument. It is far more important to be able to use a VOR than to know how it works. That being said, a VOR is a surprisingly simple device.

VOR stations are nothing more than the radio equivalent of a rotating beacon with a flashing strobe. To conceptualize the system, consider the following simplification:

The omnidirectional signal pulses as the unidirectional signal passes north.

Each VOR consists of  red unidirectional rotating beacon and a white omnidirectional strobe. The rotating beacon turns at a rate of one degree per second, so that it makes one complete rotation every 360 seconds. Every time the beacon passes through north, the strobe flashes white.

By comparing the time between beacon and strobe flashes, the airplane’s onboard equipment can determine the current radial. For example, if it you were to see a white flash followed by a red beacon 45 seconds later, then you would be on the 45 degree radial from the station.

The VOR system works in exactly this way, except that it uses unidirectional and omnidirectional radio signals instead of beacons and stobes. For accuracy, the unidirectional signal, our “beacon,” rotates at a dizzying 1,800 r.p.m.

Helpful Links:
VOR / ADF Navigation Simulator
Wikipedia’s Article on VORs

A member of an internet forum brought up an interesting question: “Why is it that you are supposed to set your altimeter to 29.92 when reaching 18,000 ft? Do you wait for ATC to tell you to do so, or is it automatic?” Read on for the low-down on transitioning to flight level flying.

Upon reaching 18,000 feet, pilots must reset their altimeters to the standard pressure setting of 29.92″ Hg. This is done to ensure altitude separation of all traffic. Furthermore, aircraft typically operate at high speeds above flight level 180, and according to Jeppesen, it would be impractical to reset the altimeter every 100 miles.

The astute reader may be wondering why the standard pressure setting is reserved for high flying traffic only. After all, if a standardized altimeter aids in altitude separation, why not apply that standard to everyone? The simple answer is that terrain does not respect standardized altimeter settings. For low flying aircraft, it is far more important to be able to precisely measure one’s height above obstructions than to ensure vertical separation of other airplanes.

To recap, pilots are required to transition to 29.92″ Hg automatically and without ATC direction as they reach 18,000 feet. Likewise, the transition to a local altimeter setting should be accomplished during the descent through flight level 180 regardless of whether ATC has advised the pilot of a local altimeter setting. After all, you were monitoring ATIS before you started down, weren’t you?

For even more information, see Wikipedia’s article on the transition altitude.