Diving is for swimming pools, Olympians, scuba and air raids. It is not for non-precision instrument approaches. At least not any more.
There was a time when conventional wisdom encouraged the practice of screaming down to the minimum descent altitudes, leveling off, slowing down, and craning thy neck in anticipation of finding a runway. The proper runway was even better.
On a typical non-precision instrument approach – one that uses an NDB, VOR, localizer or GPS track for navigation – the pilot flies a designated altitude until a certain depicted location, and then descends. The descent may only be to an intermediate altitude, or it may just go all the way down to what is called the minimum descent altitude. Once at MDA, the pilot must/shall/will level off and maintain MDA until the runway environment is in sight. Only then can the pilot leave MDA and further descend. Given that the FAA has established the MDA based on known obstacles or obstructions such as towers and terrain, it is imperative you not try to cheat the system.
Prior to the advent of the moving map GPS, the method of flying the non-precision approach was that described in the first paragraph. The reasoning was sound enough.
Without an exact knowledge of your groundspeed, winds aloft, or location relative to the runway, getting to MDA as early as possible would provide the maximum amount of time to look for the runway, especially if the ceiling and/or visibility was right at the minimum level prescribed for the approach. With DME from a VOR, this wasnt as critical as a non-DME situation, but time was still of the essence.
The result of this mayhem was a relatively high workload for the pilot in what might be a fairly stressful situation. Engine to idle, nose down, airspeed building, vertical speed building, nose up, butt against the seat, lots of power.
For the passenger, its uncomfortable, even nauseating. It also looks like the pilot doesnt really know whats going on. It can also lead to what the airlines call an unstable approach, one in which airspeed and vertical speed are all over the place.
The moving map that has been so commonplace on airliners for years, though, has made its way to even the most humble of Cessnas and Pipers. What GPS allows the GA pilot to do is know, within mere feet, the location, winds at altitude and ground speed. GPS can also tell you when you will arrive at the next point on your flight plan. For an approach, that information is golden. With a GPS, you can now plan a much more leisurely approach by flying a constant vertical speed.
Lets say that an approach has two step-down altitudes inside the final approach fix. In the old days, you would do the chop and drive routine three times. Given that a step-down altitude is going to be of relatively short duration, you can use a GPS vertical navigation feature to plan your descent from the initial altitude to the step-down altitude at a given rate of descent such that, in many instances, you never have to level off. You will pass the next fix at precisely the altitude stipulated on the chart.
This means you can keep on going to the next altitude. Plan accordingly, and you wont have to stop at that one either. The descent rate, pitch, and power settings will all remain relatively constant.
At the airlines, the pilots try to fly non-precision approaches so that the rate of descent never exceeds 1000 feet per minute; its extremely rare that they do. With the moving map and VNAV, the pilots can figure out exactly how far beyond 1000 fpm they need to go.
The next goal is to try and time the arrival at MDA with the arrival at the missed approach point. There are a couple of reasons for this. First, if the nose of the plane is even slightly below the horizon, you have dramatically increased your field of view for the search of the runway. As soon as you need to raise the nose to level off, you have taken away some of your downward vision, and that is a hindrance.
If you are the only one in the airplane and therefore have no one to help you look, it can be a huge hindrance. Any experienced instrument instructor can tell you that one of two things is going to happen to you: You will look up and either pull up right back into the clouds, or you will look up, lean forward and push over, busting right through MDA.
The second reason airlines fly this way is that if the runway environment comes in sight just as the plane arrives at MDA, then it wont be necessary to make any power changes, and probably will only need to make a minor pitch change, if any at all. The descent can just keep right on truckin.
How easy is that? It feels just like an ILS, it looks a lot more professional, and the passengers cant tell the difference. Plus the nose is down, even if only slightly, and you can see with a minimum of vertigo-inducing head movements.
Putting Theory to Work
I cant tell you how every GPS works, but most have some kind of a VNAV feature, and you should learn how to use it. Even if the unit is a hand-held not approved for IFR operations, it can still provide valuable info you otherwise would not have. Needless to say, dont be caught trying to figure out something for the first time when trying to also use it for the first time. Thats a recipe for disaster.
The GPS/FMS on most airliners have a little banana-like feature that shows where a climb or descent will be completed based on present groundspeed and airspeed. Because many airline cockpits are all glass, the crews can also create what is called a snowflake on the attitude indicator that acts just like an ILS glideslope. Now, realize that most GA planes are not so blessed, but whatever features your GPS has, you have more than you did when you did not have GPS.
Take the time feature. Even if you only have something that tells you the time and distance remaining to the next fix, you can use that to determine the rate of descent you need. For instance, you have two minutes to the next fix on the approach, and you need to lose 1,000 feet of altitude. You need to average 500 feet per minute to cross the fix at the desired altitude.
If that fix is the missed approach point (MAP), use 600 feet per minute. Granted, you may get to MDA a little early and you may have to level off, but better to be a little early with time to look than to be late and unable to land. And with real-time updates, you can change your descent rate as necessary if the winds or groundspeed change unexpectedly.
Lets look at it another way. The FARs say that you can only leave the MDA to land when you have the runway environment in sight and can make normal, non-aerobatic maneuvers to reach the runway; spins from over top the numbers are not acceptable.
Think about what this means. If you are, for example, doing an NDB or VOR or GPS approach that has the MAP right over top the runway or the airport, then it doesnt matter if you see the runway just prior to reaching the MAP. You will not be in a position to land.
Realistically, you need to be able to leave the MDA at a 500-1000 FPM clip to be able to land in the touchdown zone, especially on a short runway. Figure out – well ahead of time, thank you – where along the final approach course you need to be to safely descend and make the runway, especially if the weather is crummy. In more cases than not, it will be well prior to your actual MAP.
Take advantage of that expensive black box in your panel, and perhaps plot that point as a manually entered waypoint. When you get to, and past, your spot, give serious thought to a missed approach.
Now, for the VFR folks in the crowd. You too can use this feature to set up your descents in such a way that you can avoid energy-building dives or straining the ears of a sensitive passenger. You can even use it to help determine if you will make it to the airport sans engine. If you are flying VFR on top of some broken clouds, you can use it to help plan your descent through the cloud layer.
VNAV is one of the best features that a GPS has. It can also be one of the most underutilized. It allows you to fly a non-precision approach with ILS-like smoothness, with great situational awareness and minimal stress. It increases your appearance of professionalism, and its there. Use it.
-by Chip Wright
Chip Wright is an airline captain and a CFII.