For pilots (and their passengers), the takeoff stage of the flight is one of the most unforgiving. According to the Federal Aviation Authority (FAA), about a fifth of all general aviation accidents happen at this flight stage, and about half of these are due to pilot error. 

To help pilots, the FAA has published some guidelines that aim to make takeoffs safer. One such rule is the 30/70 rule. This might not be one of the most commonly known pilot phrases, but in general aviation, this “rule of thumb” can help to make takeoffs safer. This is really just an extension of the more commonly applied 50/70 rule (sometimes referred to as the 70/50 rule). This rule states that when planning a takeoff from shorter runways, pilots need to have hit 70% of takeoff speed by the time it’s reached 50% of its estimated takeoff distance. 

However, the 50/70 rule, as noted by the FAA, only applies to runways where there are no potential obstructions. If there are obstructions present, then this is where the 30/70 rule comes in. In this instance, then, the FAA suggests that the plane must have reached 70% of takeoff speed by the time it’s 30% through its estimated takeoff distance. If these criteria aren’t met, then the suggested action is to abort the takeoff. 

This is a simple rule that can help prevent accidents during the most critical phase of flight. Experienced pilots will tell you to apply the 50/70 rule carefully, and always to err on the side of safety, even when there are no obvious obstacles. The 30/70 rule formalizes this caution when such obstacles are present. 

The key to the value of the 30/70 rule can be summed up in one word — simplicity. The effectiveness of the rule is that it gives general aviation pilots a simple, clearly defined decision point during the takeoff phase.

There’s no guesswork and no rumbling along the runway on a wing and a prayer. Even before the plane throttles up, the pilot should have two critical metrics noted –  a point that defines the 30% distance mark and what speed the plane should be doing when they get there (70% of rotation speed). Rotation speed is the speed at which the pilot raises the nose of the aircraft and begins lifting off the runway. Various factors determine the correct rotation speed, including wind direction and speed, aircraft weight, and the altitude of the runway. 

Once these metrics are known, they essentially become performance gates, and if they’re missed, then the takeoff can be rejected when there is still room to bring the aircraft to a safe stop on the runway. 

Takeoffs from short or obstructed fields often require the airplane to be operating at its takeoff limits. These are the conditions where even small deviations can have a big impact. A sluggish engine, a change of wind direction, and softer-than-expected conditions can all suddenly change a routine takeoff into a potentially dangerous situation. These are the situations that the 30/70 rule is designed to catch early enough that pilots and passengers aren’t placed in such danger. 

The 30/70 rule may be simple, but that hides its importance. Flying is still the safest form of flying, and this simple piece of arithmetic offers pilots a clear way of keeping on the right side of this stat and, ultimately, save lives.