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The so-called 70-50 rule has popped up a few times recently on the internet. I thought it would be useful to look at this rule, explain how to interpret it properly, point out some less-helpful interpretations, and share a bit of history behind it. I use the rule frequently myself—though not rigidly.
The Rule
The 70-50 rule states:
When you have reached 50 percent of your calculated takeoff distance, you should have 70 percent of your takeoff airspeed.
In other words, it’s a quick way to check whether your airplane is accelerating at a rate that will allow you to take off within the calculated distance.
Example:
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Aircraft: T6-G
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Gross weight: 5,500 lbs
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Pressure altitude: 1,000 ft
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Temperature: 15°C
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Zero wind
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Calculated takeoff distance: 1,050 ft
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Takeoff speed: 80 mph
At 50% of the takeoff distance—525 ft—you should be at roughly 70% of 80 mph, or about 62 mph. If your speed is significantly less, it may be a good decision to abort while you still have plenty of runway left.
Notice that this method does not reference 59% of the runway or other arbitrary points. If your airplane isn’t accelerating as expected, there’s no reason to go further. Waiting until halfway down the runway at full speed to check acceleration defeats the purpose.
If your airplane isn’t accelerating as expected, there’s no reason to go further.
Measuring the Distance
You might wonder how to determine 525 ft down a runway. On paved runways with standard markings, it’s straightforward: each centerline stripe is 120 ft long, with an 80 ft gap between stripes. One pair of stripe plus gap equals 200 ft. So for 525 ft, you can count two pairs (400 ft), add one stripe (120 ft) for 520 ft, then estimate the remaining five feet.
For grass or unpaved strips, it’s trickier. My advice: calculate takeoff performance for a few different conditions, then find landmarks—trees, bushes, or other markers—to visualize the 50% distance. On unfamiliar remote airstrips, consider pacing off the distance and placing a temporary marker if possible.
A Bit of History
In the early days of the USAF, some jet aircraft had astonishingly long takeoff rolls. Pilots had little information to tell whether the airplane would become airborne at the expected distance. The solution was velocity-during-takeoff charts, which allowed pilots to check their acceleration early in the run, rather than waiting to abort at high speed with smoking brakes and tires. Over time, checking speed at roughly 2,000 ft became standard.
Mathematically inclined pilots played with these charts. Assuming constant acceleration (not exactly true, but close) and constant drag (also not exactly true), the 70-50 rule emerges as a simple, practical approximation.
A Word of Caution
Remember: you’re not a test pilot. Even if your acceleration is correct, you might not lift off at the exact POH distance. Always plan conservatively, especially for longer takeoff runs, rough terrain, or unfamiliar strips.
Always plan conservatively, especially for longer takeoff runs, rough terrain, or unfamiliar strips.
Summary
The 70-50 rule is a useful tool when applied early in the takeoff roll. At 50% of your calculated distance, if you decide to abort, you’re still moving slowly and have plenty of room to stop safely. The key is to check acceleration early, not halfway down the runway. Used correctly, it’s a simple way to enhance takeoff safety.
Editor’s Note:
Since publishing this article, several readers and experts have pointed out that the “70/50 rule” is controversial and may not apply safely across all GA aircraft. Catherine Cavagnaro’s May 2023 AOPA Pilot column, Reality Check, was referenced by several commenters and provides a thorough analysis that many pilots find persuasive. We encourage readers to review the article and always use aircraft-specific takeoff performance data, along with safety margins, when determining go/no-go decision points.
- Understanding the 70-50 Rule for Takeoff Performance - November 7, 2025
- My near miss and partial panel recovery - June 7, 2023
- What are aerobatics, and when do I need a parachute? - June 27, 2016
Isn’t 70% of 80mph 56mph?
Yes it is, I noticed that as well. Still a really good rule. Just finished my IFR and my call out is Normal. I will adopt this rule.
OOPS!! No excuse,
The so-called 70/50 rule doesn’t hold up to detailed analysis. See “Reality Check,” a column by Professor (and DPE) Catherine Cavagnaro in the May 2023 issue of AOPA Pilot magazine.
To her analysis, I’ll add that proponents of the 70/50 ratio seem never to state what 70 percent of takeoff speed is in the types of aircraft we fly, and pilots who include the “rule” in their before-takeoff briefing typically announce the ratio but omit the actual numbers. They don’t know the targets they’re looking to achieve.
For example, the short-field takeoff procedure in the AFM for the C172S assumes liftoff at 44 KIAS and accelerating to 50 KIAS at 50 ft; 70 percent of 44 is 31 knots. The similar value for 50 KIAS is 35 KIAS. The Short Field Takeoff Distance table in the Performance section of the C172S handbook assumes liftoff at 51 knots at a takeoff weight of 2550 lbs.; 70 percent of 51 knots is 36 knots.
The takeoff performance charts in the handbook for my Beechcraft A36 assume liftoff at 73 KIAS at maximum takeoff weight; 70 percent of that is 51 KIAS. According to the same table, liftoff speed at a light 2880 lbs takeoff weight is 65 KIAS; 70 percent of that value is 46 KIAS.
With those numbers in mind, consider how much time and distance a typical light GA aircraft needs to achieve 70 percent of liftoff speed. In many aircraft, the airspeed indicator has barely begun to register at 40-50 knots, and you’ll have rolled down the runway for what, 5-10 seconds?
The next time you take off, try noting the distance and time required to achieve that 70 percent value, bearing in mind that you shouldn’t be staring at the ASI as you maintain the centerline and look out for hazards ahead.
And then consider a better technique for determining a go-no decision point, as noted in Cavagnaro’s column:
“Use your takeoff performance chart to determine your anticipated takeoff distance and pad it with a value that represents degraded performance, varying pilot technique, and an engine that isn’t new. Find a point along the runway—maybe a taxiway, windsock, or similar landmark—and use this as the abort point. And don’t even attempt the takeoff if the runway isn’t longer than your computed distance plus additional necessary to safely bring the airplane to a stop.”
100% agree with Bruse Williams and Dr. Cavagnaro. In my plane (M337B/O-2A) After I feed in the front engine it is “Airspeed alive, MP, RPM, and Fuel Flow Good” about that time, I am approaching the aim point markers (captans bars) and it is “Vr, rotate.” This is not that different for the CAP C-182s I normally instruct in. Check airspeed alive, power indicators, and about that time, you should be at or near Vr and calculated take off point (plus fudge factor.) I have had to abort a couple of times (more than once to the student’s combat boots dragging the brakes) and the abort was a complete non-event. Note that I always use at least a balanced field, so accelerating to Vr and aborting is not problem.
That said, 50/70 or 2000′ may be more appropriate for turbine aircraft, or when not operating from a balanced field.
Agree Chris. Using the estimated rotation location is usually going to have or be near a better reference point, either the much more prominent marking on the runway or an adjacent reference such as a taxiway, windsock, or vasi. Estimating whether the needle will reach rotation speed at the estimated rotation point is actually a technique we constantly practice in the car. Coming to a stop, we adjust brake pressure to assure we stop at the right spot – it’s the same assessment in reverse for the takeoff roll and if there is a problem with acceleration, it will probably be obvious well before reaching the estimated rotation point. And this works the same for high altitude takeoffs where forgetting to lean can make a big difference.
The 70/50 rule cuts it too close. If you are actually at 70% airspeed at 50% length, you will take off with no runway remaining and the entire second half of the takeoff run you will not be able to abort safely. This is why there is a balanced field rule for high performance aircraft. There’s a simpler rule that is safer. If not off at 1/2 the runway length, abort. You will have room to brake without going off the end. If the takeoff performance charts indicate you need more than half the runway, you are cutting it too close and should wait for lower density altitude.
Adrian, you missed the point. Do not use 50 % of the runway length. Use 50 % of the calculated take off distance. Yes 50 % of the runway length is too far down the runway.
Thanks for this article. I had (mistakenly) thought the rule was reaching 70% of takeoff speed at 50% of the runway distance, not the planned takeoff distance. It never made sense to me why I wouldn’t worry as much about aircraft performance on a 10,000’ runway as on a 3,000’ runway, but now that I know it has nothing to do with runway length, it makes a lot more sense.
I’m glad this is getting the clarification it deserves. I’ve often heard this guideline misunderstood as 70% of takeoff speed in half the runway distance. That clearly doesn’t hold water (only achieving 70% speed half way down a 10,000’ runway should be a clear indication something is wrong in a piston single).
Keys to this guideline being helpful is (1) knowing how your plane should perform under the current conditions and (2) recognizing how much runway you have remaining to get stopped if you abort at 50% of your takeoff distance.
Airman Avolio (sorry, don’t know your rank), noticed you had your first lesson in 1946.
I.e., you are no youngster, Sir. Thank you for your time in service and for your continuing
contributions to aviation.
Ah, you noticed. What would you like to know about low frequency range approaches? It was Major, by the way.
I read Catherine’s analysis and agree with her that this rule leaves too much in doubt. An important point to me is that at the very low speeds corresponding to 70% with our 150 there is a huge relative difference between IAS and CAS you can’t believe the airspeed indicator just as it’s starting to register. If you know your airplane it’s very obvious when the acceleration is subprime. Plus, I want to be concentrating on the takeoff, not counting stripes or staring as the ASI once I check that it is registering.
This is way too much attention from your takeoff duties–especially at smaller airports for single-engine aircraft. Do your preflight takeoff calculations and make sure you have full rpm (and manifold pressure) when you release brakes or add full power. If you want to or are required to do real takeoff calculations, such as for multiengine air carriers, then use your POH for V1, Vr, and V2. Sorry, but your time is better spent looking for traffic, staying on the centerline, and monitoring your engine instruments.
Sebastian I totally agree with your assessment about this. You figuratively took the words out of my mouth. I am a relatively high time flight instructor and I also was for a limited time an FO on a 737 before my airline went bankrupt following the 9/11 attacks. So I agree with the entirety of your comments.
I once took off with a heavy passenger. Full fuel, hot and humid day, grass strip. Short grass strip…
At the halfway point, I was at my takeoff speed. It wouldn’t come off the ground. 160 hp warrior.
At the 3/4 point, I was 10 knots over my takeoff speed. It STILL wouldn’t lift off. Right then I went down into a slight dip in the runway. As I came up, eventually it was enough to get me airborne. I lifted off, and barely cleared the corn.
Note to self: Don’t try that again…
I may be the “mathematically inclined” author to which this article refers. In the column that Bruce Williams mentions, I point out several reasons the rule doesn’t make sense.
“Assuming constant acceleration (not exactly true, but close) and constant drag (also not exactly true), the 70-50 rule emerges as a simple, practical approximation.” In fact, the model that produces the rule assumes ZERO drag. Takeoff distance is also affected by winds, non-flat runways, etc. that the model fails to consider.
I sometimes hear a candidate announce that he will use the rule in the departure briefing. As he pulls onto the runway and applies throttle I ask, “What is 70% of your takeoff speed?” A sheepish “I don’t know.” is the most common response. Most unfortunate is that the rule has become a mindless part of a departure brief that apparently provides a veneer of safety.
Additionally, as an examiner, I witness that most departures exceed (many by a significant amount) the careful takeoff distance calculation performed on the ground. That means that if pilots *actually* used the version of the rule that requires 70% of airspeed at the 50% of the calculated distance point, all these would have been aborted. (I’ve never seen a candidate abort such a takeoff.) As engines wear and stop producing the power they once did, book numbers become unrealistic but that doesn’t mean the airplane is unsafe. Rather, we need to allow those comparisons to adjust calculated values and then choose a point along the runway by which time we should lift off. If that doesn’t happen, retard the throttle, take it back to the ramp and try Plan B. A reasonable rule, it turns out, is far simpler to implement.
Pilots, please stop using this dangerous rule. And, authors, please stop advocating it.
Best,
Catherine
Bill, I use the 70/50 rule on every takeoff in our P-35 Bonanza. I also calculate takeoff speed for every takeoff based on load, temp etc. Rotate speed in our Bonanza varies from about 71 to 77 MPH. Check speed is about 50 MPH to 54 MPH Takeoff distances for our Bonanza are about 1200 feet ground roll at 3,000 pounds gross weigh and 1800 to clear 50 feet. Our check speed would be 70 percent of our rotate speed of 77 MPH or 54 MPH, That should occur at 600 feet. This is quite easy to check as the first runway markers on an IFR runway are 500 feet down the runway. (Or count runway lights or use the scale on your Jeppesen Chart for runways with no markings) We’ve been doing this for 25 plus years in the Bonanza. I flew C-5’s for a lot of years. It was a hog heavy weight. We had an acceleration check speed at 120 knots for a heavy takeoff. We had to meet the time within 3 seconds to go. The time to get to 120 heavy was about 50 seconds or so as I recall. On the heaviest takeoffs, I can remember about one minute and 15 seconds to rotate, Compare that to a 737 (usually about 35-40 seconds) or a heavy 777 (48 seconds). One minute fifteen seconds is a LONG time on a runway!! Thanks for your article, John, Colonel, USAF (ret) ATP/CFII SMEL
Catherine,
Thank you for your comments. You are correct, I have never seen anyone abort using the 58/79 rule. That doesn’t mean they shouldn’t have. In addition to your reservations, there are times when I don’t feel the rule is appropriate for me. When either the Take off ground roll or the ground run to clear a 50 ft. obstacle approaches about one half of the available runway length, I reevaluate my decision to fly. I really don’t think engines get tired. I think we pilots aren’t as good as we think we are. I believe most of us cannot match the POH charts with a brand new airplane. Once again I’m glad for your effort. Thank you
Gennaro
Physics student here. No comments on the practicality of the rule, but I can explain how it’s derived. Using basic kinematics, simply compare the velocity value to its integral (the area under the triangle on a velocity-time graph). Distance is proportional to the square of time. Root of .50 = 1 / root 2 ≈ .71, so it’s actually more like the 50-71 rule. If it’s helpful, you can generate more rules by taking the square root of the decimal percent. Using a shorter distance for a check might be safer, so try 25-50 or 33-58. Additionally, if your POH has a takeoff power curve, you can use that to get a much more accurate rule for a varying acceleration.
Edit: another thing to note is that drag always increases with v squared, and the v-t graph will diverge from a straight line. This is another reason a 25-50 or 33-58 rule will be far more accurate.
The model that produces this 71-50 rule assumes no air resistance at all. If drag is proportional to the square of velocity (not always the case but for many instances, it is a reasonable assumption), it’s apparent that the drag penalty in the second half of the takeoff roll will be greater than in the first half.
Catherine