by Joseph E. Burnside
I was sitting in the back of a float-equipped Super Cub-stuffed in, is more like it-when I saw the river we were planning to fish. In my limited floatplane experience, it looked challenging enough, but there was plenty of clear water and I had confidence in the guy up front. I began preparing to get out of the Super Cub and do that for which we had flown all this way: fishing this Alaskan river for silver salmon.
Thats when the pilot, a quiet, seasoned veteran who wore his Piper like I wear socks, dropped the first notch of flaps and banked away from the river.
See something? I casually asked.
Nope; just lining up for the water, he muttered into the intercom while pulling off power and adding carb heat. I craned my neck around his seat and spotted what he was talking about.
That? Youre gonna land on that? I asked anxiously, now watching him a smooth to a mud puddle.
Busy, he didnt respond. He aimed for the edge of the mud and, after a brief splash, we were on it, taxiing for firmer ground, generally thought of as shore. I saw that the puddle was a cross between a swamp and a commercial catfish pond, rectangular in shape but only a few feet deep. Its length was ill-defined, but I have seen larger swimming pools. How are we gonna get out of here? I thought to myself. More importantly, we had three others in a Cessna 180, also on floats, now flying a pattern to this same mudhole. Were going to have to part out that airplane to get it home.
We soon limited out, cleaned our fish and packed for the hour-long flight back to base camp, a large, secluded lake. As we finished loading, the Super Cub pilot unpacked his rifle and walked over to me.
Here, he said, handing me the rifle. Youll need this; there are bears in the area. Ill be right back.
With that, I stood on the firmer ground, watching the Cessna and the Super Cub, each with two aboard, get on their steps and lift off the mudhole. After about 30 minutes (it only seemed like an afternoon) of being the only hot lunch in the immediate area, the Super Cub splashed back in, we stashed the rifle and headed for camp.
Thinking about this later, it made perfect sense. Since either airplane could only carry two people apiece out of the mudhole, I got the short straw to remain behind and catch the last flight out. Funny, but I dont remember drawing that straw.
I also dont remember anyone consulting a chart or doing a takeoff performance calculation. Instead, these guys not only knew their airplanes but they had been in and out of that mudhole before. They knew the performance they needed-the Cessna was well below gross-and what they had to do to get it.
We Cant Plan Takeoffs
In 1999, 20.6% of 1877 general aviation accidents occurred during takeoff. Thats slightly more than one a day and second only to landings as the most accident-prone phase of flight.
One example is the Cessna 172B pilot whose plane wouldnt climb after takeoff from an Oregon airport with a 5550-ft density altitude and all four seats filled. After settling back to the runway, the airplane accelerated and finally lifted off. But climb performance was poor and the pilot elected to make an off-airport precautionary landing. The Cessna was damaged, but no one was injured.
And then theres the guy with a 17-day-old Private pilot certificate who attempted taking off a Cherokee 180 at a 7650-ft density altitude. According to the NTSB, the airplanes nose pitched up, the airplane yawed left and right, and … control was lost … the airplane impacted runway lights and came to rest after colliding with a tree. Duh. The combined weight of all the occupants was about 585 pounds. About 30 gallons of fuel was aboard.
We cant do every takeoff by instinct. Instead, we need to paw through the charts for some of them to compute our expected takeoff performance. But first, we need to learn which takeoffs require checking performance and which dont.
Instinctive Planning
In the two cases above, the pilots either thought they did not need to plan their takeoffs or planned poorly. Their instinct may have told them that they didnt have a problem. On the other hand, our two Alaskan pilots used their instincts to calculate takeoff performance-I got to hold the rifle. Its easy to say their instincts were better; the results were takeoffs after which the airplane could still be used. However, their experience levels were higher, also. So, right away, weve identified one factor in deciding whether we need to perform a takeoff calculation: experience. Of course, experience can tell us when we dont need to perform a takeoff calculation but it can also tell us that if we offload a passenger, some bags or some fuel-or a combination-that the takeoff will be incident-free.
But how can the pilot still hoping to get those years of experience instinctively know when to offload stuff, get out the performance charts, or both? The answer is to calculate performance for takeoffs outside your normal operations.
If you base on a 2000-foot sea-level grass strip and regularly get your grossed-out Bonanza on and off it with no problem, then you may not think a 2500-foot strip in the mountains with trees at each end is a big deal. Run the numbers, though, and youll find its a lot more marginal.
Similarly, if you regularly fly without bags or passengers, once you load it up-even at your home plate-looking over the performance charts to make sure the familiar 3000-foot strip of pavement will be adequate wont hurt a thing.
The point is to not only know your airplane but to know when youre outside of your normal operations and about to try something you havent done. Thats when you need to calculate takeoff performance.
Data Collection
After deciding to plan a takeoff, we need data. We need to know how high we are, how hot it is and how much runway we have. We also need the height of any obstacles and their distance from the end of the runway. Finally, we need to know how heavy our plane is and if there are any issues with it-like a dragging brake or soft tire-that will affect our takeoff performance.
Most of the time, what we need can be obtained from the local ATIS or automated weather reporting broadcast: Just write down the numbers and plug them into the takeoff chart. But if were at Not Even A Telephone, Tenn., we need to improvise. If this is the case, first thing should be to climb in the airplane and note the OAT. Then, set the altimeter to 29.92 inches and note the elevation. This gives us our pressure altitude. Dont forget to re-set the altimeter back to field elevation.
Somewhere, we should be able to find the runways length. If that info isnt available (how did you know the runway was long enough to land on?), youll need to step it off for situations involving a real, honest, max-performance takeoff. Dont worry; it wont take long and you probably need the exercise.
While youre stepping it off, you should also estimate the distance from the runways end to the nearest major obstacles as well as their height. Believe it: If you cant find the runways length anywhere in your charts and information, the airport is sufficiently off the beaten path that it has to have obstacles.
Numbers Running
Plugging the numbers into the chart is something every student pilot has done, but rated pilots-like you and me, who fly from 5000-foot paved runways in relatively low density-altitude conditions-probably havent done this lately. The graphic on the previous page presents some takeoff performance results for a few popular aircraft. The thing for pilots to keep in mind is the effect of weight and density altitude on such performance.
For our examples, well use a more-or-less standard summer day at an average airport. The temperature is 90 degrees Fahrenheit and the field elevation is 1000 feet. The altimeter setting is 29.85 inches. Oh, and the windsock is as limp as that fresh shirt you pulled out of the duffle this morning-you wont get any help from the wind.
Youve loaded your 1977 Cessna 182Q to its maximum gross takeoff weight (MGTOW) of 2950 lbs. with fuel, bags and mothers-in-law. Under these conditions, it needs a ground roll of approximately 880 feet to get airborne and a total of 1690 feet to clear a 50-foot obstacle.
Thats respectable runway performance-this is a Skylane, after all-but it also presumes a perfect airplane and perfect technique. To achieve this performance, the tires must be properly inflated, the brakes cant be dragging and the O-470 under the hood needs to crank out all 230 of its ponies. These numbers also presume lifting off at exactly 49 KIAS and accelerating to 57 KIAS at the proverbial 50-foot obstacle, plus the other steps Cessna specifies for short-field takeoffs.
Real-World Numbers
While mere mortals can achieve close-to-book results after an hour or so of practice, trying to do it right the first time with your new girlfriend aboard might be tough. When was the last time you tried max-performance takeoffs at gross weight? Are you confident your Skylanes engine doesnt have a soft cylinder and is performing as God and TCM intended? Right. To be on the safe side, and to compensate for a worn airplane and bad technique, add 50% to the numbers.
Our new runway requirements include a takeoff ground roll of about 1320 feet-the length of a drag strip-just to get all the wheels off the ground. To climb all the way up to 50 feet AGL, youll need some 2535 feet. If youve got 5000 feet of dry, paved runway, its not a problem. If, however, youre trying to get out of that 2500-foot field you popped into in cooler weather, you might get up close and personal with that oak tree at the end.
But, lets see what would happen if you leave your spouses mother behind. For our second condition, lets take that same 1977 Skylane on that same hot day, but offload people, bags and/or fuel so that our MGTOW is down to only 2400 lbs. Under those same conditions, our ground roll is shortened to about 548 feet, while the tree-avoidance distance is 1043. Adding the standard 50% fudge factor, our real-world numbers come in at about 822 and 1565 respectively. Thats much more comfortable performance on a 2500-foot strip.
The 182s takeoff performance, as well as performance for a Beech Bonanza A36 and a Piper Arrow IV, are charted on the preceding page. Note that under the conditions selected-which are not that uncommon this time of year-none of the aircraft can clear the 50-foot obstacle at the end of a 2500-foot runway using real-world numbers.
Go/No-Go
Once weve run the numbers and determined that we have an adequate safety margin-the distances available should be at least 150% of the book-we need to think about how well know if things are going as planned and what well do if they arent. In other words, we need to determine an abort point-if were not airborne and climbing by the time we reach it, we close the throttle and stand on the brakes. Lindbergh used one when he left for Paris; so can we.
One way is to look up the landing distances required for the proposed operation based on altitude, temperature, etc. But that can mean a whole new set of calculations and chances to bend metal.
Instead, a common practice is to mark or note the runways halfway point. If you have not achieved 70% of the necessary liftoff speed by that halfway point, youre not accelerating fast enough and will run out of runway, ideas and brakes all at the same time. To be on the safe side, you might want to make that 100% of the liftoff speed.
Once reaching your abort point and deciding this takeoff isnt going to work out as planned, smoothly close the throttle, firmly apply the brakes and maintain directional control. Dont touch any other controls, especially the flaps-you can grab the gear handle and really stop short. As the airplane slows, smoothly add back elevator pressure to keep weight on the mains and improve braking efficiency.
Once youve stopped, taxi back to the ramp and offload the mother-in-law. Give her your rifle and tell her youll be right back.
Also With This Article
“Takeoff Performance Examples”
