“During the takeoff roll, when the airspeed had reached about 65 knots,” the pilot “looked up and saw an animal in front of the airplane.” The pilot said he “instinctively pulled up to avoid the animal;” the “airplane rolled and the left wing impacted the ground. The airplane then veered to the left, across a road and impacted trees.” The four aboard received “minor” injuries and the airplane was “destroyed.”
–from the National Transportation Safety Board, preliminary report
Rosanne Roseannidanna warned us years ago: “It’s always something.” As pilot-in-command you may have meticulously planned your takeoff, and used the five-point method of predicting and evaluating takeoff performance. No takeoff will ever go exactly as predicted in the Pilot’s Operating Handbook. Like the animal on the runway, there are often some outside influences, some extenuating circumstances that result in the true takeoff performance achieved — and whether you will have to abort a takeoff.
What are some of the external factors that may affect takeoff performance, meeting your “acceleration” target (see Abort, Abort! — Part 1) and perhaps the need to abort?
- Wind. Headwinds will generally decrease the ground roll and reduce the distance required to clear that mythical 50-foot obstacle. Crosswinds may make directional control more difficult (especially if the winds are variable or gusty), and might increase the ground roll and obstacle-clearing distances if the crosswind correction requires large, drag-producing control deflections. Tailwinds will increase the ground roll and obstacle-clearance distances, seemingly out of proportion to the strength of the wind (a little tailwind makes a big difference).
- Density altitude. Hot air is less dense than cold air. Engines “breathe” air — when less is available, engines produce less power. Wings produce less lift (for a given ground speed) and propeller blades are less efficient. It’ll take more ground roll to get the “normal” volume of air over the wings for takeoff lift, and it’ll take longer to cover that ground than it would at sea level because of reduced engine and propeller efficiency. Once aloft, reduced engine power and higher “true” speed (for a given indicated air speed) reduce the climbout angle.
Insider’s Note: Even turbocharged engines are less efficient as the density altitude increases. Turbochargers may boost air pressure to make up the difference, but this compression heats the induction air, making it less dense as it enters the cylinders — reducing power. Intercoolers (radiator-like devices often fitted to turbocharged engines to cool this compressed air) aren’t very effective at low airspeeds, so they have little effect on engine power at takeoff … and may even reduce power further by restricting combustion airflow. Propeller blades are also less efficient as the density altitude increases, regardless of turbocharging.
- Weight. The lighter the airplane, the better the performance. Conversely, the heavier the aircraft, the longer runway you’ll need to become airborne, and the greater distance you’ll need to clear obstacles. If the airplane’s loaded over the maximum takeoff weight, you have no way of knowing just how significant that degradation may be … or if the airplane will fly at all.
- Center of gravity. Even within the published center of gravity (c.g.) limits, the further the c.g. location toward the forward end of the envelope, the harder it’ll be to rotate for takeoff, and the greater control deflection needed to hold a climb pitch attitude. Hence, a forward c.g. loading (within limits) may require a longer takeoff roll as the airplane accelerates to a point the elevator has the oomph to lift the nose; holding climb pitch requires a greater-than-normal elevator deflection, increasing drag and decreasing climb performance. If the pilot doesn’t exert the extra force necessary to hold climb attitude, the airplane will climb out at a reduced angle, increasing obstacle clearance distance. Conversely, a within limits c.g. near the aft limit may actually reduce takeoff and obstacle-clearing distances slightly, but beware — loadings make the airplane less stable and more likely to “over rotate,” or establish an excessive angle of attack (nearer to stall) at rotation. Of course, a c.g. location outside the approved limits (forward or aft) may so radically affect stability and climb performance as to be suicidal.
- Runway surface. The smoother the runway surface, the closer to “book” takeoff you might expect. In fact, most Pilots Operating Handbook (POH) takeoff performance is predicated on a “clean, dry, level runway surface.” Any contamination (bumpy or uneven runways; wet or snow-covered pavement; gravel, grass or other unpaved surfaces) will significantly increase the ground roll requirement and, consequently, the distance required from the beginning of the takeoff to the 50-foot obstacle-clearing height.
- Runway slope. POH performance comes only on a level runway. Any runway upslope increases distance requirements. On the other hand, a downslope reduces takeoff distances.
PERSONAL EXPERIENCE: I used to fly a Beech Baron from a short runway with a steep runway slope. I found — after cautious experience at reduced airplane weights and in varying wind conditions — that the effect of taking off downhill was usually more beneficial than taking off uphill into the wind. Such was standard procedure for all multiengine airplanes at my home airport; because landing airplanes usually landed uphill, in the opposite direction, it took good visual scanning, radio technique, and otherwise standard traffic patterns to assure safety at the never-busy-but-steadily-used airport.
- Airplane condition. Takeoff performance tables result from tests of a new airplane. If your is a little out of rig, or the engine is less than perfect; if a propeller blade is nicked, or contamination like frost, snow, or even dust or dead bugs is on the wings, the airplane won’t perform exactly as planned. Aircraft age alone may reduce performance.
- Pilot technique. Similarly, POH flight testing was done by a professional test pilot. If you’re not one yourself, expect less-than-perfect takeoff performance. And the charted performance expectation is based on a specific takeoff technique. Called the “associated conditions” on the Takeoff Performance Chart, this technique often closely mirrors the “short field takeoff” we all learned as student pilots — full power before brake release, recommended flap settings, and rotation at a precise airspeed. Any other technique will result in different (probably reduced) takeoff and climb performance.
- The unexpected. That animal on the runway. An engine “hiccup” on takeoff. Splashing through puddles or bouncing over rough pavement. A door popping open, or other demanding distraction. Any of these can (and unfortunately, often do) show up as contributing factors to takeoff-related accidents.
RULES OF THUMB — DEALING WITH THE “KNOWNS”
Most extenuating circumstances are “knowable” — you ought to be able to anticipate them before you begin your takeoff roll. With anticipation comes the ability to compensate somewhat … or at least know what performance degradation to expect. Let’s look at some “rules of thumb” for dealing with the “known” extenuating circumstances:
- Wind. Consider the effects of wind on your takeoff and climb performance. Use wind effects from your POH Takeoff Performance Chart, if available. If not, a “rule of thumb” might be to decrease the runway and obstacle clearance distances by 10% for each nine knots of headwind, or increase distances by 10% for every two knots of tailwind component (from the Cessna 172RG POH).
Estimating Headwind and Crosswind
You can estimate the headwind and crosswind components of a reported wind as follows:
|Angle between wind
|Multiply by to
|Multiply by to
|–from William Kershner’s The Advanced Pilot’s Flight Manual|
- Density altitude. POH performance charts should also provide correction figures for nonstandard density altitudes. Compensate for density altitude when making your takeoff calculations. Remember also that high density altitudes usually require manual leaning to achieve a takeoff mixture setting. Check the POH “Normal Procedures” section for information on optimizing mixture setting for high-altitude takeoffs.
- Weight. The lighter, the better, so far as takeoff performance goes. All else being equal, plan to take off at the lightest weight that fuel requirements (including a generous reserve) dictate. If you’re taking off from a short field and will need a lot of fuel for your trip, consider taking off with a reduced fuel load and planning an early stop to top off en route.
- Center of gravity. Load the airplane so the c.g. is well within limits. Do you need to sit down with the POH and pencil out a loading report every time you take off? Not really — after you’ve done it a few times, you’ll know what’ll work (and what WON’T) under normal circumstances. When you’re flying out of a shorter runway, or at a higher density altitude, or loaded heavier than is normal FOR YOU, well, that’s when you do need to compute your center of gravity location and other performance factors before the flight.
- Runway surface. Wet, contaminated, or unpaved runways will degrade takeoff performance. But by how much? The Cessna 172RG POH says “for operation on a dry, grass runway, increase distances by 15% of the ‘ground roll’ figure.” Kershner gives us further guidance:
|Runway Surface||Increase Distance by|
- Runway slope. This one is very subjective because the slope of the runway can be very gentle, or very steep. Kershner suggests a 10% additional correction to computed figures for an uphill run, and confirms my experience that often it’s better to take off downhill with a tailwind than to try it uphill with a headwind. Best bet is to be very cautious and conservative, and make any uphill takeoffs at as light a weight and as cool a temperature as possible. Pick an expected rotation point for your “acceleration target,” and immediately abort the takeoff if you haven’t reached flying speed by that point.
- Airplane condition. Assuming the airplane is properly rigged and the engine is in good condition, your defense against “airplane condition” factors is to properly inspect the aircraft before flight, and assure the airplane is free of ice, snow, frost or other contamination. Close inspect the propeller for nicks (ask your mechanic to show you what is, and what is NOT, airworthy in the way of a damaged propeller). Keep the airplane clean and remove insects from the leading edges of the wing — dirt and bugs alone probably won’t keep an airplane from flying, but combined with other factors it’s one more level of risk you don’t need to take.
- Pilot technique. The POH lists a technique required to get “book” performance. If your planned takeoff is short enough that you need to calculate distances for the specific flight (the runway and conditions aren’t “normal” to you, so your previous calculations aren’t valid), then review AND USE the POH takeoff technique. This probably includes powering up before brake release, flaps settings, rotation speeds, and initial climb speeds or pitch attitudes.
- The unexpected. Here’s where you earn your keep as a pilot. If you’ve properly accounted for all the “knowns” for your takeoff, and for some reason can’t attain one or more “takeoff targets” … if that animal wanders onto the runway … if the engine hesitates … if a door pops open or any number of other unexpected distractions crop up, you’ll need to be ready to abort the takeoff. You’ll need to do it promptly and safely, avoiding (or at least minimizing) injury and damage.
BOTTOM LINE: With all this to think about and evaluate before taking off, it’s a wonder any of us ever actually leave the ground. In practice, it’s an easy habit that begins long before you get in the airplane. Learn and use the tools you need to anticipate and compensate for reductions in aircraft performance — and to immediately detect when you need to throw in the towel and abort a takeoff in progress.
Next time we’ll finish up by reviewing considerations for safely aborting a takeoff.