Have you ever thought about what you would do if the engine wasn’t producing enough power to get you airborne during takeoff, or quit immediately after takeoff? How would you determine whether the engine is performing as advertised in the POH, during those first critical seconds of the takeoff roll? Do you check the oil temperature, oil pressure, and tachometer to verify the engine is performing as expected? Do you check the airspeed indicator to make sure it’s registering correctly? Do you know where your abort point is on the runway if any of the aforementioned parameters are abnormal?
When performing a routine touch and go for practice, how closely do you pay attention to where you actually touch down, and how far down the runway you are at the point of liftoff? After the touch and go, do you have enough runway straight ahead to land if the engine were to quit immediately after liftoff? Where will you land if the engine quits and there’s no available runway straight ahead? If the answer is “I don’t know” to any of these questions, you have some homework to do. Read on.
Let’s begin by taking a look at a concept known as accelerate-stop distance. The FAA Airplane Flying Handbook, Chapter 12, Transition to Multiengine Airplanes, defines accelerate-stop distance as “the runway length required to accelerate to a specified speed, experience an engine failure, and bring the airplane to a complete stop.” But the handbook doesn’t provide any guidance on this for single engine pilots.
An obscure FAA fact sheet titled Pattern Precision, published in 2020, suggests the following: “When planning takeoff from short unobstructed runways, establish a landmark at 50% of your calculated takeoff distance. When reaching that landmark, you should be at 70% of your rotation speed. If not, abort the takeoff and reduce weight or wait for more favorable wind and temperature conditions.”
This so-called “70/50” (or “50/70”) rule has been well known to bush pilots for decades, and can be a lifesaver when dealing with high density altitude situations, as in this example. But are there other reasons why a pilot might not reach that key speed by the predetermined abort point? What if the engine isn’t quite producing as much power as advertised? Check out this pilot’s story:
In a single-engine Cessna trainer, we can use a combination of accelerate-stop distance and the 70/50 rule to determine the point at which we must abort the takeoff and have sufficient runway straight ahead to stop. If we combine the takeoff and landing data from the POH, we can estimate how much runway we would need to accelerate to takeoff speed, get airborne, and then land straight ahead if the engine quit immediately after takeoff.
Let’s study the data for our beloved 1969 Cessna 150J, N60254, which is by all accounts the weakest performer in the fleet in terms of climb performance and cruise airspeed. The POH advertises a takeoff ground run of 735 feet, with 1,385 feet required to clear a 50-foot obstacle; and a landing roll of 445 feet, with 1,075 feet required to clear a 50-foot obstacle. If we put these two numbers together we can estimate that if we start from a stop at the threshold (using all available runway), accelerate to takeoff speed and get 50 feet in the air, we would have traveled 1,385 feet at that point. Then, if the engine quits, we could descend, land and come to a stop from that 50-foot height in another 1,075 feet, resulting in a total distance traveled of 1,385 + 1,075 = 2,460 feet. Knowing that the runways at our home airport are approximately 4,000 feet long, we can calculate that we must begin the takeoff roll no further than 1,500 feet from the beginning of the runway in order to have enough room to execute this maneuver if needed.
The reality, though, is we rarely begin our takeoff roll from the beginning of the runway. More often than not, when winds are favoring Runway 32 or 23, we depart from an intersecting taxiway (E or F, respectively) and have approximately 3,600 feet of runway available from that point. This is still more than our 2,460-foot requirement, but remember the POH numbers were based on a brand new airplane with a brand new engine. Also, we still need to know how to visually identify the abort point.
The 70/50 rule says we should have achieved 70 percent of our takeoff speed by the time we reach 50 percent of our takeoff ground run. Using the N60254 data, our estimated ground run at sea level on a standard day is 735 feet, so half of that is approximately 368 feet. Let’s round that up to 400 feet, which is a much easier distance to visualize on the runway. The FAA standardizes the distance between runway centerline stripes at 200 feet, from the beginning of one stripe to the beginning of the next one (including the blank space between them). So tell yourself that you’ve got two runway centerline stripes to get to 70 percent of your takeoff speed.
What speed is that? The POH suggests that during the takeoff run, we “lift the nose wheel at 50 mph.” So 70 percent of 50 mph is 35 mph. What does that look like on the airspeed indicator? Not much, since the airspeed indicators on both Cessna 150’s bottom out at 40. So let’s just say we need the needle to reach 40 by the 400 foot mark in order to continue. This is why a pilot should call out “airspeed alive” and “engine instruments in the green” as soon as full power is applied and the takeoff roll has begun. Check that the engine is making the required RPM at full power (remember, you did your runup at 1700 RPM, not full power).
How much runway would we need in front of us to safely execute a touch and go? We never come to a complete stop after touchdown, so our takeoff run will presumably be shorter than advertised in the POH because we are already rolling and have some airspeed, but not enough for takeoff. Conservatively, we could state that we still need the same 2,460 feet of runway remaining straight ahead in order to execute the “go” part of the touch and go, and still have enough runway ahead to land and stop if the engine quits immediately after takeoff (i.e. at or below 50 feet). If we round the 2,460 up to 2,500, and subtract that from our full runway length of approximately 4,000 feet, we can state that we must not touch down any further than 1,500 feet from the beginning of the runway while executing a touch and go. Good practice for the FAA Private Pilot practical test would suggest aiming to touch down no further than the 1,000-foot marker, but for safety, touching down as close to the numbers as possible.
The next time you depart Runway 23, take a good look straight down after you pass the fence that bounds the airport property. What’s down there? Where would you land if the engine quit? The higher you are at that point, the more options you have. The closer to the beginning of the runway you are at the moment of liftoff, the higher you will be upon reaching the edge of the airport property.
There’s an old saying in aviation that goes something like: There are two things that are completely useless to a pilot: runway behind him, and air in the fuel tanks. Don’t give away precious runway when executing a normal takeoff or a touch and go!
|Cessna 150||Cessna 152||Cessna 172|
|Takeoff: Ground Run||735||755||890|
|Takeoff: 50’ Obstacle||1,385||1,390||1,625|
|Landing: Ground Run||445||485||540|
|Landing: 50’ Obstacle||1,075||1,215||1,280|
|50% of Takeoff Ground Run (Rounded Up)||400||400||450|
|Vr (Liftoff Speed)||50||50||55|
|Airspeed Required to Continue at 50% Mark (Rounded Up)||40||40||40|
|Runway Required for engine failure at 50’ (Rounded Up)||2,500||2,600||3,000|
|Min RPM for Takeoff (Static RPM, Full Throttle)||2500||2280||2300|