I've helped many pilots make the transition from four-seat, trainer-type airplanes into high-performance, retractable-gear airplanes and powerful light twins. If you've made that transition yourself, you probably remember zooming up through your desired (and perhaps air traffic control-assigned) altitude as you grew used to the new airplane, and the challenge of accurately transitioning to level flight from descents in an aerodynamically slick airplane.
Have you ever come up on altitude so quickly you had to give the controls a good push or pull to level off? With experience you may be able to nail altitude this way, but it still results in a bad ride for your passengers ... people who sometimes you need to impress so they will tolerate your flying habit. What do you think of the crew when sitting in an airliner's cabin and feel some positive- or negative-g when the crew makes an altitude change? Right -- you're unimpressed with their technique. There's opportunity for "style points" on level-off from climb and descent, finesse that makes the final transition almost imperceptible to nonpilots along for the ride.
Sometimes it seems that the airplane just won't accelerate leveling out of a climb, or you can't get it to slow down in descent and when leveling at approach or pattern altitude.
Part of the transition's challenge is ergonomic. Differing seat mounting and cowling angles may mean the way level flight "looks" is different, which takes a little time to get used to. Higher power and the performance that brings obviously play a part in the ease of leveling precisely on altitude. (I'm sure I'd take some time to get precision out of a jet.) More often than not, however, imprecise level-off control comes (I think) from poorly learned habits -- or no habit pattern at all. Last month in Leading Edge #2 we talked about establishing standard operating procedures for various phases of flight. This month let's look at a specific SOP I use for precisely leveling off from climbs and descents. Maybe it's something you'd like to add to your own technique.
Robert Sumwalt, long-time accident investigator and aviation safety author and now vice chairman of the National Transportation Safety Board (NTSB), coined a term some time back he called the Altitude Critical Area (ACA). For Sumwalt the ACA is defined by altitude above ground level, an area history shows to be of high risk for aircraft accidents mainly due to workload and distractions (and, to no small amount, proximity to the ground). I expand on the concept of ACAs to include the last 1000 feet of climb or descent before leveling off -- also an area of high workload and distraction and potentially hazardous if the target altitude is missed, especially in descent.
Why add these level-off transitions to the ACA? Because these are the times to employ some ACA-like techniques:
Accurately and smoothly leveling off from a climb comes through a series of actions that begin well before the desired altitude. I teach beginning the process of level-off at 1000 feet below your altitude target.
When leveling off from climb to cruise, in many airplanes you'll experience a dramatic change in trim. A stable airplane will attempt to maintain the indicated airspeed for which it is trimmed. If you climb at 110 knots, for instance, and your cruise indicated airspeed will be 140 knots, in the transition between climb and level you'll need to make a big nose-down change in trim or the airplane, trying to maintain trimmed speed, will want to continue to climb. The trim change is not immediate, either. Give forward pressure on the yoke and trim off that pressure. The airplane accelerates and "passes" that intermediate trimmed speed. Hold more pressure and re-trim. It may take several minutes before the airplane reaches its final speed, during which it will not be precisely trimmed at any time. Power changes may make this even more dynamic.
You can see why concentration and deferral of tasks is important. You'll also find the trim change required at any given point is less if you start the level-off process at 1000 feet below altitude -- you begin the acceleration, and the trim change, sooner and arrive on altitude at something closer to cruise indicated airspeed.
There's an archaic notion that airplanes, like speedboats, can get "on the step," or in a low-drag configuration that results in higher cruise speeds, by climbing slightly above altitude and then diving down in the transition. Although this may get the airplane up to cruise speed a little faster, this myth has been pretty much debunked and the final cruise speed will be the same. A common older practice of reducing power just as you level off (instead of accelerating under climb power, then pulling back) may have contributed to this notion.
Another "step" to accelerating to cruise speed that may have more validity, however, involves controllable-pitch propellers ... especially three-bladed props. Although higher prop speeds generally generate more thrust, they also create more drag than the same prop at a lower rpm. In some airplanes this may be noticeable. For instance, an A36 Bonanza with a three-bladed propeller will accelerate smartly on level-off through about 130 knots indicated airspeed, but then the rate of airspeed increase noticeably slows. I think the propeller is creating a "drag wall" through which it takes more time to accelerate. I have found that, if I reduce rpm as the airplane is reaching 130 KIAS, the rate of acceleration continues unhindered up to cruise speed. In fact, if I want to cruise at a high rpm, I can pull the prop back a couple hundred rpm at 130 KIAS, accelerate to near final speed, and then advance the propeller again to sneak up on those last few knots. This is likely very airplane-specific, so the prop drag may not be evident in the airplane you fly.
It's even more important to avoid descending through an altitude; after all, down is where the ground is. As with climbs, I teach a descent level-off technique that begins 1000 feet above my desired altitude.
Depending on the speed at which you descend, and your desired airspeed at the new, lower altitude, you may experience a dynamic trim change similar to leveling off from climb, except in reverse. This can be doubly dangerous because, during the transition, the airplane may be trimmed for a faster indicated airspeed, meaning the airplane's tendency will be to continue descending below what may be a minimum obstacle-clearance altitude. Again, ACA discipline and a gradual level-off beginning 1000 feet above altitude help you avoid this trap.
The Aviation Safety Reporting System notes it receives "several thousand" reports of altitude deviations each year. The vast majority of these "NASA reports" come from professional flight crews flying airline and corporate equipment -- which are flown on autopilot much of the time. The gist of these reports is that "altitude busts" come in three forms:
"George" (a now-anachronistic nickname for autopilots) is not pilot-proof. By far most autopilot-driven altitude deviations result when the pilot (or crew) did not correctly program the altitude preselect feature, and the aircraft climbed or descended through an assigned altitude. Altitude preselects have been available in high-end, single-pilot airplanes for years, and undoubtedly many altitude busts (and perhaps even controlled flight into terrain accidents) have had misprogrammed autopilots as a contributing factor. If a crew of two (or three) professional pilots can input the incorrect data and not discover the error before blowing through an altitude, it seems even more likely this could happen to a single-pilot operator who may not fly or train as often. The lesson? Input autopilot data, then ritualistically confirm that the data you have input is correct.
Lack of autopilot monitoring by the crew resulting in an altitude bust has been reported frequently as well. Alone in the cockpit? To employ an over-used Ronald Reagan quote, "Trust, but verify." Monitor the autopilot closely to confirm it does what you have programmed it to do. Here the concept of the ACA will help you prioritize your actions when nearing your desired altitude, weighted heavily toward confirming the level-off occurs as planned. An autopilot is a workload-reducing device, not a replacement for the pilot-in-command.
Occurring much less frequently, a malfunction of the autopilot system is known to result in altitude deviations as well. Here again, confirmation of data input (and acceptance by the autopilot) and close monitoring will help you discover when George is on holiday. Avoid the temptation to sit back and "see what the autopilot is doing" if it is not maneuvering the airplane as expected. Be ready at all times to take over manually, especially if the autopilot is somehow missing a level-off.
Leveling off, whether from climb or descent, by hand or using the autopilot, is a dynamic change in attitude, trim and performance. During the transition the airplane will naturally be out of precise trim, with a tendency to "bust" altitude without active intervention. Being "behind the airplane" makes altitude deviation even more likely, and invites jerky actions that are imprecise and uncomfortable to passengers. Automation carries an inherent risk of operator or mechanic failure that demands close monitoring by the pilot.
There will be times when you need to modify your technique to fit traffic or terrain demands. But when you have the option, an SOP that begins the level-off transition 1000 feet early and includes specific actions at 100 feet to go and when reaching target altitude will help you make safer, smoother, more accurate and better-performing level offs.