The Take on Takeoffs
The takeoff is a maneuver that's treated as a no-brainer by many pilots, but there's a whole lot more to a properly-planned takeoff than meets the eye. AVweb's Linda Pendleton discusses what pilots should think about before every takeoff, how to know if a rejected takeoff is warranted, and how to deal with takeoff emergencies if it's too late to abort.
Over the years, I've had the opportunity to hang around a lot of airports and participate in many hangar-flying sessions. (That's more rotten coffee than you even want to think about!) One thing I never hear is a pilot talking about going out to practice takeoffs. It's always landings. Nobody brags about his or her takeoffs. It's always landings. Takeoffs are the forgotten maneuver. It's just taken for granted that after the first hour or so of dual that EVERYONE can do a takeoff and no further thought on the subject is required.
Well, as you may have imagined by now, it's just not quite that simple. It's true that most takeoffs don't seem to require the fine judgment and coordination that landings do. Also, who (besides a blimp pilot) ever heard of an emergency takeoff? The consensus of opinion seems to be that takeoffs require little or no planning and that just about anybody can finesse one. Almost true, but stand behind the fence someday and watch down the runway as folks takeoff. You'll almost always see the tail swing to the right and the wing drop even if just momentarily before everything straightens out and "flies right!" Maybe a little more practice is in order.
However, that's not the major failing of most pilots in regard to takeoffs. There's a major lack of planning and a certain complacency associated with takeoffs that isn't warranted. After all, at no subsequent point in the flight will the airplane be heavier and using all of its available power to fly and climb at a low speed. Yet lots of pilots simply turn onto the runway, advance the throttle and launch into the blue. Ask most of them how much runway the airplane used to accelerate to takeoff speed or whether the acceleration was normal, and you'll usually be met with a blank look.
Doing It By The Book
The tried and true Skyhawk is familiar to most of us, so let's see what the book figures are for this plane. (See Figure 1.) The key to understanding and using any aircraft performance charts is in understanding how the manufacturer constructs them. Notice the conditions listed in the upper left corner of the chart. They specify 10° flap; full power before brake release; a paved, level, and dry runway; zero wind; and liftoff at 51 KIAS. Just how you do every takeoff, right? "But," I can hear you protest, "I don't need to do short field takeoffs, my home runway is 7,000 feet long." Nevertheless, this is the only takeoff performance chart Cessna provides for this airplane. Since you won't duplicate the conditions listed, the chart is useless.
The book says that for a sea-level airport at 20° C it will take 980 feet on the ground and 1,745 feet to clear that mythical 50-foot obstacle. There are not a lot of runways that are too short for that. Again, I hear you tell me about your 7,000-foot runway. Well that 7,000-foot runway is one of the problems. All that concrete in front of you tends to lull you into complacency.
Remember Palm 90? That Air Florida 737 crashed into the Potomac River. The engine gauges erroneously gave the proper readings, yet the engines were not putting out the anticipated power. The acceleration took longer than it should have. (The co-pilot pointed this out several times and then capitulated to the captain who saw no problem. The aircraft staggered into the air, stalled and crashed.) The crew had no guidance readily available in the cockpit to judge the acceleration much like you in the 172. That's where the 7,000-foot runway becomes a potential problem. The 172's engine would have to be pretty sick to not get off the runway in 7,000 feet so you are less likely to discover a problem before you leave sweet mother earth. I think you'll agree that the time to find out about engine problems is before departure, not 300 feet in the air over houses. (That is NOT fun. I had it happen once, and it'll really got me thinking!)
Takeoff Reality Check
So, what can we use to determine whether performance is healthy? It's a pretty good bet that you're not going to make every takeoff with 10° flaps or apply full throttle before brake release, so the book numbers aren't going to be achievable. What's a pilot to do? It just so happens that you DO have an instrument in your cockpit that will help you gauge your engine's performance and, actually, the Palm 90 crew had it available to them, too, but apparently nobody taught them how to use it.
That instrument is a plain old clock!
You've done this before time x speed = distance. Also, time = distance/speed. Okay, I can just hear you saying, "Wait a minute! That won't work because the airplane is accelerating." You're right, but as it turns out, you can use the average speed and get close enough for our purposes. There is one complication, however. Speed is in knots (nautical miles per hour) and we're going to be looking for feet and seconds to work with here. Just use this handy table. I've taken the liftoff speed in knots, halved it to get an approximate average speed, and then converted it from nautical miles per hour to feet per second. (For those who are interested that's speed x 6000 = feet per hour/3600 = feet per second.)
This is inexact. (John Lowry could probably give us some exact formulas here, but I'm just not up to calculus right now.) We're just trying to establish some guidelines here. Of course, you know that the propeller has certain inefficiencies that are not constant across the speed range and the acceleration itself is not a totally straight-line function, but this will be close enough for us to use.
Now you need to know what your ground roll should be for your takeoff. You can do this two ways. First, you can take the ground roll given in your POH and add a fudge factor to account for your differing technique. The other method is to have someone note exactly where you lift off using your normal technique and measure the distance. No matter which method you use, you're going to have to do this for varying temperatures and altitudes to get a good idea what your performance actually is and to set some guidelines for judging future performance.
Let's take an example from the Cessna 172R Short Field Takeoff Distance chart. How about a 1000' MSL airport at 10°C. The chart says the ground roll should be 1,000 feet. Divide that 1,000 feet by the Average Speed (Feet Per Second) listed for your liftoff speed (rounded up). That will be 51 knots for our 172R, which we'll round up to 52 knots and use the table to obtain an average speed of 43.3 feet per second. Divide 43.3 feet per second into 1,000 feet of ground roll and you get 23.1 seconds.
So it should take you about 23.1 seconds to accelerate to liftoff speed under these conditions. That's a number you can find in the cockpit. You can even set a countdown timer to measure it.
What makes this better than the numbers found in most POHs? Well, humans have been around for a long time and we move at a normal speed of about four miles per hour with very short bursts to about 15 mph. Our senses have a hard time measuring anything that is faster than that and one of the things we're really poor at is judging distance during acceleration. The only numbers you get in POHs tell you that you should be off the ground in X number of feet. Unless there are distance markers on the runway you're using or you are familiar with the airport, you have no reliable way of judging the acceleration of your plane during the takeoff roll.
If it takes you significantly longer than your calculated number to accelerate to liftoff speed, it would be prudent to apply the brakes, taxi back to the ramp and figure out what's going on. That's where the 7,000-foot runway is handy it gives you space to do this without having to change brake pads before the next takeoff. Remember, however, that the 7,000-foot runway can also allow you to take an inordinate time to accelerate without it being apparent because the end lights are not rushing up on you.
Plan For The Worst
So, what do you do if in spite of all your planning the engine does start to give up the ghost off the departure end of the runway? Well, the time to think about that was during the preflight planning before you even untied the airplane. Pick out a place to put an airplane in an emergency and do it for every takeoff. It's easy at your home field because you are familiar with the geography, but the information is available for other airports, too. You can always ask the locals where they would go in the event of an engine failure. The perceptive ones will have some good places picked out. The place that is probably not available to you is the runway you just departed. There has been much discussion about the turn back to the runway, and many words written advocating this maneuver. Make your own judgment based on your experience and abilities, but in most cases at less than pattern altitude, it's a risky maneuver.
Altitude and airspeed are your friends on initial climb after takeoff, yet many pilots sacrifice precious knots and feet by dragging the gear for an inordinate amount of time. What's inordinate? Anything after a positive rate of climb is established and shows on the VSI and altimeter. I suspect the old saw about leaving the gear hanging until there is no runway left to land on was devised by a flight instructor who wanted to keep new retract pilots from inadvertent gear retraction before a climb was established. Positive rate, gear up. Eliminate the drag and increase your speed and rate of climb. If the engine quits, the least of your problems is getting the gear back down. True, there are times you may want to leave the gear out a little longer taxiing through snow and slush comes to mind but as a practice, positive rate, gear up.
There are exceptions to every rule, of course. For example, retracting the gear on a Cessna 337 Skymaster results in a huge initial increase in drag as the gear doors open early in the retraction cycle, so the POH advises against retracting the gear until obstacles are cleared. Know your airplane. MB
If all the math has made your eyes glaze over, you can judge your takeoff performance by picking out a prominent landmark alongside the runway at a known distance. You can use any number of things taxiways, windsocks, wind tees every airport has something. Just pick something that is about where you should be ready to lift off. If you aren't up to speed by then, stop. There can be any number of reasons for sluggish acceleration and most of them are best figured out on the ground.
To sum it all up, know how long (time or landmark) it should take you to get to liftoff speed, clean up the airplane after liftoff to get the best performance (and that includes keeping the wings level and the ball in the center!) and know where you plan to go if the worst happens. You'll probably be like most pilots and never need this planning, but if you do need it, YOU'LL NEED IT!