AIM ¶ 7-6-8 — Runway Half-way Signs
AIM 7-6-8 explains the 70% liftoff airspeed rule of thumb at runway half-way signs on unimproved airports — when to abort takeoff and key limits.
In Plain English
At unimproved airports, runway half-way signs give you a visual cue to judge whether your takeoff roll is going as planned. The rule of thumb: by the midpoint of the runway, your airplane should have reached 70% of liftoff airspeed. If it hasn't, abort the takeoff — you likely won't have room to lift off in the remaining runway.
Key caveats every student pilot should know:
- Airspeed indicators in small airplanes aren't required to be accurate below stall speed and may be unreliable at 70% of liftoff speed.
- The rule assumes a uniform surface. Puddles, soft spots, tall/wet grass, or loose gravel can slow acceleration — even after the midpoint.
- It only addresses the runway needed for liftoff, not obstacle clearance. Obstacles in the climb path require additional distance to accelerate to Vx.
- It doesn't account for upslope or tailwinds, which demand even greater acceleration.
- Inspect the entire runway before takeoff.
This rule of thumb does not replace the AFM, FARs, or manufacturer performance data. The signs also help you judge position during landing rollout. Note that there is no FAA standard for the sign's appearance.
AIM Source Text
FAA AIM ¶ 7-6-8When installed, runway half-way signs provide the pilot with a reference point to judge takeoff acceleration trends. Assuming that the runway length is appropriate for takeoff (considering runway condition and slope, elevation, aircraft weight, wind, and temperature), typical takeoff acceleration should allow the airplane to reach 70 percent of lift-off airspeed by the midpoint of the runway. The “rule of thumb” is that should airplane acceleration not allow the airspeed to reach this value by the midpoint, the takeoff should be aborted, as it may not be possible to liftoff in the remaining runway. Several points are important when considering using this “rule of thumb”: Airspeed indicators in small airplanes are not required to be evaluated at speeds below stalling, and may not be usable at 70 percent of liftoff airspeed. This “rule of thumb” is based on a uniform surface condition. Puddles, soft spots, areas of tall and/or wet grass, loose gravel, etc., may impede acceleration or even cause deceleration. Even if the airplane achieves 70 percent of liftoff airspeed by the midpoint, the condition of the remainder of the runway may not allow further acceleration. The entire length of the runway should be inspected prior to takeoff to ensure a usable surface. This “rule of thumb” applies only to runway required for actual liftoff. In the event that obstacles affect the takeoff climb path, appropriate distance must be available after liftoff to accelerate to best angle of climb speed and to clear the obstacles. This will, in effect, require the airplane to accelerate to a higher speed by midpoint, particularly if the obstacles are close to the end of the runway. In addition, this technique does not take into account the effects of upslope or tailwinds on takeoff performance. These factors will also require greater acceleration than normal and, under some circumstances, prevent takeoff entirely. Use of this “rule of thumb” does not alleviate the pilot's responsibility to comply with applicable Federal Aviation Regulations, the limitations and performance data provided in the FAA approved Airplane Flight Manual (AFM), or, in the absence of an FAA approved AFM, other data provided by the aircraft manufacturer. In addition to their use during takeoff, runway half-way signs offer the pilot increased awareness of his or her position along the runway during landing operations. FIG 7-6-1 Typical Runway Half-way Sign
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Oral Exam Questions a DPE Might Ask
Q1What is the runway half-way sign rule of thumb for aborting a takeoff?
Per AIM 7-6-8, if the airplane has not accelerated to at least 70 percent of liftoff airspeed by the midpoint of the runway, the takeoff should be aborted, because liftoff may not be possible in the remaining runway.
Q2What limitations should a pilot consider before relying on the 70% rule of thumb?
Per AIM 7-6-8, airspeed indicators may not be accurate at speeds below stall; the rule assumes a uniform surface (no puddles, soft spots, tall grass, or gravel); it only covers runway needed for liftoff, not obstacle clearance; and it doesn't account for upslope or tailwinds. It also doesn't relieve the pilot from complying with FARs or AFM performance data.
Q3Besides takeoff, how can runway half-way signs assist the pilot?
Per AIM 7-6-8, runway half-way signs also provide increased awareness of the airplane's position along the runway during landing operations.
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