Cruise through aviation forums on the Internet and you’ll find descent planning is a popular topic. Ask how people plan their descent and answers range from, “I wait for ATC to give me lower,” to a series of calculations, with a lot of rules of thumb in between.
Actually, decent planning at its finest really isn’t as simple as the former, but doesn’t need all those in-flight calculations, either. The variability comes in the type of flying, and other considerations like comfort, terrain and wind.
Consider This
Bring the power back to idle and hang a few things out in the slipstream, and most airplanes can hurtle earthward at up to about 4000 feet per minute without exceeding the limitation of red line on the airspeed indicator.
But, who wants to do that? Surely, your passengers would be uncomfortable with a view that feels like it’s straight down. If you’re not pressurized, you certainly don’t want to subject anybody’s ears to all that painful popping. Even if your plane is pressurized, if it’s piston-powered, you probably don’t want to subject the cylinders to the shock cooling. So, yes, there are a lot of considerations.
A rule of thumb we use at the airline is a maximum of 10 degrees nose down. I’ll reach that occasionally when terrain, ATC or forgetfulness keeps me too high for too long. But, even with that modest sounding 10 degrees, I’ve occasionally gotten some sideways comments from a flight attendant or even a passenger.
Anyway, the lesson from this is that a little negative deck angle goes a long way toward eroding the often tenuous grasp some folks have on comfort with flight.
In fact, I remember one time when I had a pilot from another airline in front with us in the cockpit jumpseat. The plane I fly has a sight picture that already feels a bit nose down, and the attitude on a normal approach accentuates that. To the uninitiated, this can seem rather dramatic. It was his first time up front in one of these beasts. It was visual, and I could tell he was among the uninitiated by the way he was squirming with increasing urgency as we neared the runway. Breaking sterile cockpit, I turned around and mentioned that we really weren’t going to land like a lawn dart and this was a normal sight picture. He relaxed. Some.
Rounding out passenger comfort is rate of descent. About 500 feet per minute for unpressurized cabins is a practical maximum that folks can take in stride. Any more than that and you risk passengers’ ears not clearing adequately.
With piston engines, you should consider shock cooling. Some folks think this is bunkum, citing basic trainers that go from full power to idle all day long with pattern work. But experts I respect who know tons more than I do, say it’s a real consideration. For me, a little planning allows me to gradually reduce power in a descent just in case it is a real problem. And if it’s not? Well, the worst I’ve done is smooth out the ride, so no harm, no foul.
Wait for ATC to Assign Lower
Many of us actually do, uh, plan our descents this way, but there are obvious flaws. First is that you’re in control and responsible for the proper outcome of a flight, not some guy/gal in a dark building possibly hundreds of miles away.
The controller is simply not responsible to proactively descend you to land. Sure, ATC knows where you’re landing, and they should be developing a plan for your descent. But conflicts, weather, terrain and other considerations may get in the way of a plan that’s optimum for you. In these cases, you get left too high for too long, or worse, get descended too early and you sacrifice a lot of the efficiencies gained at higher altitudes.
You need your own plan. Of course, you can’t execute your plan without ATC permission when you’re IFR, but if you don’t have a plan, you’ll only get what ATC gives you. If ATC’s plan differs from yours, you can abandon your plan and go with theirs, or try to negotiate with the controller to get something that more closely resembles your own plan.
OK, so now even though you might simply be waiting for ATC, you do have an idea of when you’d like to start your descent. Let’s look at your options in case ATC’s plan differs significantly from your own.
Last month in “A Little Help from ATC,” Ian Blair Fries covered a number of ways to work with ATC when they want a descent that’s sooner and/or lower than you’d like. I won’t repeat all that information here, but your arsenal contains some simple tools. If you’re below the flight levels and it’s VMC, you can simply cancel IFR and descend at your pleasure. If that’s not an option, request that the descent be at pilot discretion. Failing that, ask for a crossing restriction as the controller might not necessarily need that altitude until you cross a particular fix. Your last option is to explain why you’d rather not descend just yet and possibly work out some alternative.
With the opposite situation—ATC won’t give you the lower you want until much closer to the airport—you’ve also got some options. Of course, cancelling IFR remains as a possibility, again if the weather is VMC and you’re below FL180.
You could ask for a reroute. This might be useful when you’re kept too high for terrain or traffic. Sometimes just a few minutes off your course gives you adequate clearance from the conflicting traffic or places you over lower terrain.
If these don’t work, and you’ve pleaded your case to the controller, prepare for a more rapid descent than normal, because you’ll still eventually have to descend. If you’re ready for a rapid descent, you might be able to make the airport without any fancy maneuvering.
I often fly between San Francisco and Medford, Oregon and almost always get essentially a direct route. That direct route is nearly aligned with the preferred runway, so we can usually count on a straight-in approach to the airport. Unfortunately, Medford has mountains near the airport and terrain keeps us far too high for far too long.
The first time I went in there, I ended up doing a few 360s in the valley to get low enough to land. Being a person who occasionally learns from painful—and embarrassing—experience, I’m now ready. As I approach that ridge line, I’m already as low as ATC allows. Then I slow. I’ll slow all the way to flap and gear speed. Fortunately, the ridge is not above the maximum gear altitude.
Then, as soon as ATC lets us descend, I throw out the boards (spoilers), drop the flaps and gear and do my best falling rock imitation. I can usually almost get to the minimum altitude at the final approach fix.
OK, we’ve gotten this far and I’ve ranted about having a plan, but we haven’t discussed making a plan. Let’s do that.
Just Use 3:1
One rule of thumb commonly used by pressurized or relatively low performance aircraft, is to plan a simple 3:1 descent. (It’s not really 3:1 because the units are incompatible, but go with me here.) For every 1000 feet to descend, plan on travelling three nautical miles. So, if you’re at 10,000 feet above your target, you’d want to start your descent 30 miles out.
We need a little math to calculate a descent rate, but you’ll only have to do the math once. Say your groundspeed is 180 knots. That’s three miles a minute. (One mile a minute is 60 knots; 180/60=3.) In that one minute, you travel three miles. In that three miles you lose a thousand feet in that one minute, so you’ll need—put down the calculator; this one’s easy—1000 feet per minute.
If your groundspeed is but 120 knots, that’s only two miles a minute. With a three mile descent profile to lose 1000 feet, in two miles you’ll need to lose two-thirds of that 1000 feet, or 667 feet. So, at 120 knots, our 3:1 plan needs 667 feet per minute. That’s not too bad without pressurization.
If that selected groundspeed is your target true airspeed and you’ve got a tailwind, you’ll need a smidge higher descent rate; with a headwind, you can reduce your descent rate a bit.
500 Feet per Minute
As we discussed above, 500 feet per minute is about the maximum unpressurized descent rate for ear comfort. So, we’ll plan our descent rate at 500 feet per minute.
At 500 feet per minute you lose 1000 feet in two minutes. With 10,000 feet to lose, start down 20 minutes out. If you know how far out you are in time, this is all you need. But, it’s also a good idea to translate that to a distance as a backup.
How fast are you going? If it’s 120 knots, that’s two miles a minute, meaning you’ll start down at 40 miles from the airport. If you’re streaking along at 180, three miles a minute, you’ll want to start your descent 60 miles out.
Of course, there are a couple caveats. If your cruise speed is 120 knots and you nose over into a descent at 500 feet per minute, your airspeed is going to pick up. You may find yourself covering the ground faster and should have started your descent somewhat sooner. So, when picking an airspeed, think of the airspeed you’ll achieve in the descent. Further, it’s not really the airspeed that matters. It’s the groundspeed. For the purposes of this rough calculation, though, assume they’re equal and just bump up your descent rate a bit for a tailwind or reduce it a bit for a headwind.
Shock Cooling
Flying a piston? Worry about cooling. You don’t want to just chop ‘n drop the way you might do with a turbine.
Managing your engine temperatures is as much art and technique as it is hard science. An engine generates heat and the airflow through the cowling carries that heat away. If the rate of generation is lower than the rate of dissipation, your engine will cool. In general, that’s a good thing. Cool engines are happy engines.
But, it can be a bad thing if your engine cools too rapidly from increased airflow through the cowling, risking cylinder damage or at least increased cylinder wear and shortened life. If you’re running a turbocharged high-performance six-cylinder engine, it can cost $50,000 or more to overhaul and have a life of less than 2000 hours. Even at 2000 hours, that $50,000 engine costs you $25 per hour. If, through harsh treatment you only get 1000 hours, well, you’ve doubled your cost to $50 per hour for the engine. Be gentle and don’t take chances.
The common rule of thumb here is to first close the cowl flaps if they’re not fully closed at cruise, then reduce power by not more than two inches of manifold pressure in not less than one minute. I start a timer each time I reduce power on descent. When the timer reaches a minute, I further reduce power and reset the timer. If you’re running at, say, 30 inches of power, and you need about 18 inches in the pattern, you’ll easily reduce the power that 12 inches in only six minutes. Plan your descent to include power reductions and the speed reduction that will go along with the lower power and lower altitude.
Consider This, Too
Of course, most navigators offer descent planning. But, like most other aspects of using the box, you’ve got to first program it right, and then understand what it does.
Say you program your descent rate at 500. The box typically uses your current groundspeed to calculate when to start down. As we discussed above, if you start down at near-cruise power, you’re going to gain speed. The box will have to call for a higher descent rate to stay on profile. So, some thought from you is still necessary, even if you’d prefer to use the box’s brains instead of your own.
Let’s talk about airspeed again. If it’s only going to take you six minutes to bring your power back, and you’ve got a 20 minute descent, you can blast downhill at cruise power for the next 14 minutes. Or can you?
Remember that redline? Remember your maneuvering speed? Sure, you can fly all day long at the top of the yellow, just below red line, as long as it’s smooth. But get a bump, and you should immediately slow to maneuvering speed. That reduction in speed won’t be easy because you’ll either have to reduce your rate of descent, bollixing up your descent plan, or reduce power faster than you should considering the health of your engine. So, research the ride on the way down before you commit to a specific plan.
Oh, and one more word of caution is in order. Remember your target altitude is seldom zero. Say you’re cruising at 10,000 feet and you’re landing at Kearney, Nebraska, where the ground is at 2131 feet. In round numbers you’re only descending 8000 feet. But wait, there’s more. If you’re doing anything but planning a straight-in approach, you’ll have to do some maneuvering, so include the pattern at 1000 feet AGL, or possibly the IAF for your likely approach. See, yet more planning is required.
Bottom line, here, is that the planning isn’t difficult, but it is necessary. Like most things in aviation, a little advance thought can make it seem easy. Lacking that advance thought and waiting for the controller to tell you what to do will eventually bite you in the empennage.
Frank Bowlin is a lazy ATP. Whether he’s flying an airliner at his day job or his personal airplane, he usually gets away with the 3:1 rule, as long as he’s mindful of the other factors.
