As summer takes hold, the days are getting long. Theyre also getting hot, which means the other thing that will be getting long is takeoff distance.
High temperatures – and therefore high density altitudes – affect every airplane, single-engine or multi, piston or jet. Higher elevations and bigger payloads only make the problem worse.
In a single-engine airplane, the loss of engine power makes the next step pretty easy to determine. If the engine failure happens on the ground, you will stop. If the failure happens in the air, you will land. Sir Isaac Newton assures us that this is so. Such outcomes are non-negotiable and cannot be changed.
In a multi-engine plane, it is not always so simple. Its true that, for piston twins, the loss of an engine on the runway will indeed be cause to abort the takeoff in all but the most unusual circumstances, simply because the other engine is not really powerful enough to get the plane off the runway, especially on a hot summer day.
All pilots, however, can take a page from the multi pilots book by adopting a sound risk management tool: calculating the accelerate-stop distance.
Multi-engine pilots in piston aircraft are trained that an engine failure at any speed on the runway is cause to stop. The accelerate-stop distance is defined as the distance it will take to accelerate the plane to its liftoff speed and still brake to a complete stop should an engine fail at rotation.
As the temperatures and density altitudes get higher, it is indeed possible that the total accelerate-stop distance will exceed the length of the runway. The accelerate-stop distance shows the pilot if its possible to stop on the runway remaining if an engine quits during takeoff.
If the numbers say that it cannot be done, the pilot is left with a few choices: Go anyway and hope an engine does not stop, reduce the load to reduce the distance required, or cancel the flight.
The rules are a bit different for turboprops and jets, especially those certificated for use as airliners. In these birds, the second engine is truly considered to be a spare.
In a turbojet airliner, pilots calculate three critical speeds before every takeoff: V1, VR, and V2. They consider outside air temperature, airport elevation and the actual gross takeoff weight. If, during the takeoff roll, an engine fails before reaching V1, the pilot aborts the takeoff and knows its possible to stop the airplane on the remaining runway. Once past V1, the airplane is flyable and an engine failure is considered an in-flight emergency. The pilots know the plane will continue to accelerate to rotation speed and will fly out at V2, the takeoff safety speed.
In making the calculations, airline pilots take into consideration such factors as wet runways, runways contaminated with slush or snow, and any mechanical problems that may affect performance, such as inoperable thrust reversers. If necessary, the airline will reduce the fuel load or carry fewer bags and/or passengers to meet the runway requirements.
Build Your Own
Piston planes certainly dont have the options available to airliners, but knowing how much runway it would take to stop is a useful bit of information. The pilots of singles are not required to calculate accelerate-stop distances, and technically they do not even exist. However, we can fudge the numbers a bit and come up with a ballpark figure.
As you doubtlessly recall, youre required by the FARs to calculate your takeoff and landing distance prior to any flight. Few pilots do, unless pondering a flight into a truly short – and unfamiliar – airstrip. That might be worth reconsidering.
For instance, the runway at my home airport on the Chesapeake Bay measures 2,900 feet. In the summertime, even with a pressure altitude of sea level, a fully loaded Cessna 172 on a 90-degree day will require approximately 995 feet of runway to become airborne. It will need 1,810 total feet to clear the FAAs 50-foot obstacle at the end of the runway.
The same plane, landing at the same 2,400 pounds, will need 570 feet of runway to roll to a stop after touching down. If coming over the 50-foot tree, the airplane requires a total of 1,325 feet.
Based on ground roll numbers alone, the airplane needs 1,565 feet of asphalt to accelerate to takeoff speed and then stop. Based on this arithmetic, there is plenty of room to accelerate to takeoff speed, lose the engine, and roll to a stop on the remaining runway.
If, however, the plane has gotten airborne, things get dicier. Assume the plane climbs to 50 feet, loses the engine and immediately begins to descend. (It wouldnt really start descending immediately, but well say that it does for the purpose of this exercise.)
Now, we are looking at 1,810 feet of takeoff distance plus 1,325 feet of landing distance for a total of 3,135 feet of total runway used.
In this case, the 172 heads off the end of the runway. I hope you packed a bathing suit, because using runway 29 at Bay Bridge (W29), you will be rapidly approaching a watery finale. Note that I did not include a headwind, because the winds at W29 are almost always a direct crosswind. Remember also that these numbers do not take into consideration any deceleration glide that will occur after engine failure. The true distance is likely to be a bit longer.
Keep in mind also that the data used to create the charts in the POH were generated by a test pilot in a brand-new airplane with brand-new brakes, tires, engine and prop. The pilot was well versed on the short-field takeoff techniques used to create the POH takeoff data. Likewise, on landing, he had been able to practice the short-field technique several times before beginning to log data.
In the POHs for twins, where an accelerate-stop distance is given, remember that the pilot knew the engine was going to fail.
You, on the other hand, are probably flying an older airplane that has a few thousand hours under its belt. The tires and brakes may be a bit worn. The prop may be nearing overhaul time and the engine may not have the umpf it once had. If your airspeed indicator is out of calibration, you may use up extra runway while accelerating to the proper indicated speed.
Last but not least, you will not know when the engine will quit. This is not a big deal while still on the ground but, rest assured that to you, it will be a huge deal – and surprise – when airborne. Any idea, Mr. Yeager, where you will land? Where you can land?
Even if youre in a multi and have the POH accelerate-stop distances, remember those caveats still apply. The first time you experience an engine failure on takeoff, your heart will jump as the airplane kicks and yaws from side to side. On your first couple of tries, you will spend more time trying to straighten the plane out than you will on stopping it.
Other factors to consider are grass runways, which penalize both the Warrior and the 172 in excess of 30 percent extra distance, runway slope (10 percent for both planes), tailwind components, and wet or snowy runways. Move the same 2,900-foot runway and 2,000-pound airplane to Asheville, N.C., at 2,100 feet of elevation and the picture gets worse.
All these things must be considered when you start loading up for that long-anticipated trip with your family or friends or piling in business associates to head for a conference. Some flight instructors advocate padding performance numbers by 50 percent to take into account all the variables than can affect performance. Some use more, others less.
Single-engine pilots do not need to calculate accelerate-stop numbers, but it is food for thought, a worthwhile exercise in risk management. You need to know when you start the takeoff roll that you can stop on the runway remaining should it become necessary.
Do the book work and add a substantial fudge factor. You may not have textbook accelerate-stop distance, but you do have numbers that you can use. So use them when necessary, and be as safe as possible. Besides your engine, what else do you have to lose?
-by Chip Wright
Chip Wright is an airline captain and CFII.