Your instrument training was all about the physical tasks of flying approaches, missed approaches and holds. Your CFII didn’t spend a lot of time on en route descents, or an efficient way to get prepared for those close-in, “slam-dunk” procedures…you’d pick up all that with experience flying “in the system” after passing your instrument check ride. At least that was the unspoken understanding.
Trouble is, you’ve been flying IFR for a while, even completing a couple of instrument proficiency checks since passing the practical test, but those quick IPCs focused on the same terminal procedures you’ve been flying since your instrument training began, and you’re starting to wonder if there’s an easier, more efficient, better way to get from cruise altitude to the ground. The made-up-as-you-go method you’ve come up with in the absence of an IFR internship like the corporate and airline pilots get early in their careers just isn’t getting it done for you anymore.
The Scenario
You’ve been zooming along in cruise for a couple of hours, monitoring your position and your autopilot, pleased with the smoothness of your flight and the unusually good groundspeed of a brisk tailwind. The engine gauges all read precisely what you know to be normal for this particular airplane. The unexpected groundspeed means your fuel state is even better than you’d planned. You pick up the ATIS for your destination airport:
Wichita Mid-Continent Information Tango, 1500 Zulu. Winds 010 at three knots. Sky condition: Few clouds at 300, ceiling 700 broken, 1100 overcast, visibility one and one-half miles, mist. Altimeter 29.93. ILS 1L, 1R in use, aircraft landing and departing runway 1L and 1R. Notice to Airmen, taxiway Bravo 3 closed, south half of the west cargo ramp closed. Advise on initial contact you have Information Tango.
An instrument approach is in your immediate future.
All is going quite well; now it’s time for the workload to pick up, for you to transition from cruise through descent, approach, a missed approach and diversion if needed, and eventually to a landing.
Top of Descent
One concept that appears in the lexicon and technique of airline pilots but rarely makes it into the training of non-professional general aviation is the concept of “top of descent.” Usually abbreviated TOD or T/D, top of descent is a point, in many cases calculated by a flight management system (FMS), at which an airplane should begin descending from cruise flight in order to arrive, after a constant rate of descent, at the initial altitude for an approach or, in the case of a VFR or visual approach, at the traffic pattern altitude.
The nearly ubiquitous GPS navigator now makes this calculation easy for most general aviation pilots. With the descent profile function that allows the pilot the select a desired rate of descent (usually 400 or 500 fpm in unpressurized airplane, potentially steeper in pressurized types), you can plan to arrive at a selected altitude over a desired location (say, at a charted transition or approach altitude two or three miles outside an initial approach fix). As you descend, you can monitor the vertical descent required (sometimes called the vertical profile) window to see if your current rate of descent will get you to the altitude and location target you’ve selected.
This is a great crosscheck because the GPS calculates TOD based on your groundspeed at cruise altitude. Winds will change, and generally lessen, as you descend; true airspeed will decrease for a given indicated airspeed at lower altitudes. This becomes a significant factor in turbocharged and turbine airplanes descending from the flight levels (or even the low teens). So pilots in these airplanes may delay their descent a couple of minutes from the point the computer commands beginning the descent, and adjust vertical speed as needed, based on their navigator’s GPS-generated vertical profile as the airplane loses altitude.
We did this manually before the advent of GPS and FMS, of course. And it’s not difficult (it’s like dusting crops, kid). Say you’re at 7000 feet, and you want to be at 1800 feet two miles prior to the initial approach fix (IAF) for a terminal procedure. That’s a little more than 5000 feet of altitude to lose. At a sedate and passenger-friendly 500 fpm descent rate, it will take 10 minutes to make this descent. If your groundspeed is, say, 150 knots, or 2½ nautical miles per minute, you’ll travel 25 miles during this descent. Begin your constant-speed descent 25 miles from the IAF. Your true airspeed will decrease during the descent, so the 25 miles approximation is very close even though you need to lose an extra 200 feet (about 24 additional seconds of descent) to reach 1800 feet. So, even if you are using DME, a request for radar information from a controller, or good, old-fashioned deduced reckoning, you can make a good judgment of when to begin your descent from cruise.
Of course, this all goes out the window if ATC does not permit you to begin your descent precisely when you wish. In most cases, you’ll have to make a request of controllers to time your TOD to the optimal position. Along the way, there may be an ATC-imposed altitude restriction, whether for traffic, terrain or another operational reason. Regardless, TOD is an important concept for all pilots, VFR as well as those flying on an instrument flight plan, because there are specific things you should do at or just prior to top of descent. The checklist in the sidebar on page 17 is a good place to start.
During Descent
During your descent, continue to monitor the vertical speed required, and adjust it as needed to stay on your desired profile. You should already be at or below your airplane’s maneuvering speed (VA) if turbulence is reported or expected, but if you are not at VA or less already and encounter unexpected turbulence, immediately reduce power to reduce airspeed. You don’t get any “free bumps” to alert you to slow down and avoid overstressing the airplane, so on hot summer days, in mountainous areas, when even “fair weather” cumulus clouds are present, or when surface winds exceed 30 knots, plan on making your descent at or below VA.
Some other thoughts on what to do during your descent for the approach are in the sidebar at left.
Cleared for the Approach
There are three items of information that are critical for you to know without looking at the approach chart, from the point just outside the Final Approach Fix (FAF) to the Missed Approach Point (MAP). These items are the:
1. Minimum altitude you may fly on the approach, whether it’s the decision height, decision altitude or minimum descent altitude. Because the altitude for a given approach on a given day will vary depending on the equipment you’re using, your airplane’s approach category for the speed you’re flying, and whether you will be flying a circling approach, it’s vital to pick the proper altitude from the many options on the approach chart during your briefing prior to TOD, so you can commit it to short-term memory and confirm it just before reaching the FAF.
2. Distance you’ll fly from the FAF inbound to any step-down fixes and the MAP.
3. Direction and altitude you’ll fly for the initial segment of the missed approach procedure, should you have to miss the approach.
You might even consider, as I do, writing the approach critical information (altitude, distance and missed, or “ADM”) on a sticky note and sticking it on the panel where it is near your primary scan. This is for those last-minute doubts as you near the missed approach point, so you can ease your misgivings without diverting your attention to trying to pull the information from a busy approach chart. The graphics on page 19 highlight these items and how to organize them for two different approach plates.
When you are given a vector to intercept the final approach course inbound, then, or when you are cleared to join some other segment of an approach course or hear the words “cleared for the approach,” accomplish these actions:
1. GPS MODE—VLOC or GPS, as required for the approach type
2. HSI or VOR HEAD Inbound or Transition Course—CONFIRMED
3. Heading Indicator—CONFIRMED
4. Approach critical items—REVIEWED (Course, Altitude, Missed)
Approach/Missed Approach With most of the work done prior to reaching the FAF, and a quick confirmation of the course, altitude and direction of the initial segment of the missed approach procedure, all you have to do on the final approach segment is fly the airplane and descend at the proper time and place. You’ve already set and checked everything, and committed the vital ADM information to memory, so you can devote all your mental bandwidth to flying the approach or closely monitoring the autopilot as it does so.
The missed approach procedure is a part of the approach, merely an extension of the safe routing for when you cannot fly the final, visual portion of the procedure. When properly briefed, you won’t have to look down to figure out what to do if you decide you need to abort the procedure, or if you reach the MAP and do not have the runway environment in sight from a position where you can make a normal descent to the touchdown zone. Commit to the missed, and you know the initial direction and altitude to fly.
After reaching the altitude at or above which you can make turns, you can take the GPS out of SUSPEND (or similar) mode, if using one, and glance at the approach chart to review what you’ve earlier briefed. You might even make a second sticky note for the full missed approach procedure so it, too, is in your primary scan for this high-workload time, although I caution against putting too much on to many sticky notes—things can easily get too cluttered.
All Together
From the FAF to the MAP, and beyond, your goal should be to focus your attention solely on flying the procedure…the sort of thing that is the focus of most instrument rating training and instrument proficiency checks. The difference is that, having created and used a few short checklists for Prior to TOD, Crossing TOD, Descent and Approach/Missed Approach, you will fly the approach with the confidence that you’ve not forgotten something vital.
0513-TOD-to-MAP-and-BEYOND.pdf
