Hold-Short Decision at a Busy Controlled Aerodrome
You are pilot-in-command of a single-engine piston aircraft, VFR, taxying for departure from a controlled aerodrome on Runway 27. Tower has cleared you to 'line up and wait' Runway 27 at intersection Bravo, which leaves 620 m of runway available. Your aircraft's take-off distance required (TODR) from the full-length threshold at the current conditions is 540 m, and the take-off run required (TORR) is 410 m. Density altitude is 1,400 ft, surface wind 250/08 kt, temperature +24 C. As you read back the clearance, Tower adds: 'Expect an immediate departure, traffic on a 3-mile final, 140 kt groundspeed.' You have not yet entered the runway. The aerodrome's declared distances show TODA 1,180 m and TORA 1,180 m from the full-length threshold, with no displaced threshold.
1. Using the declared distances and your performance figures, determine whether accepting the intersection departure is lawful and safe. Show your calculation and state the decision.
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Step 1 - Convert to available distances. From intersection Bravo you have 620 m of runway available. Because the declared TORA and TODA are both 1,180 m from the full-length threshold, the intersection gives you a TORA and TODA of 620 m (no clearway or stopway is added at the intersection, so the figures do not improve).
Step 2 - Compare with required distances. Your TODR is 540 m, which is less than the 620 m available, so the take-off distance requirement is satisfied with 80 m margin. Your TORR is 410 m, also less than 620 m.
Step 3 - Apply the safety factor. The margin is only about 15 percent on TODR. That is thin for a departure where Tower expects an immediate roll and where you will be starting from a standing position with no rolling start. The wind is nearly aligned but only 8 kt, and density altitude of 1,400 ft already inflates your figures; any further temperature rise or a wind shift reduces the margin further.
Decision: The departure is lawful on the numbers, but the combination of a 15 percent margin, an immediate-departure expectation and a standing start makes it operationally marginal. The correct judgement call is to decline the intersection and request the full-length threshold, or accept only if you are confident the aircraft will be at full power before brake release and the tower can give you a realistic gap. Declining is never penalised; accepting a thin margin under time pressure is.
2. Tower's traffic is on a 3-mile final at 140 kt groundspeed. Estimate the time you have and explain how this should influence your acceptance of the clearance.
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Estimate the time. 3 nm at 140 kt groundspeed is 3/140 hours, which is about 0.0214 h, or roughly 77 seconds. In practice the aircraft will decelerate on final, so the real time to the threshold is somewhat longer, perhaps 85-95 seconds, but you should plan on about 75-80 seconds as the conservative figure.
What that time must cover. Within that window you must: enter the runway, complete the pre-take-off checks and line-up, obtain and read back a take-off clearance, spool up, release brakes, accelerate, rotate and climb clear of the runway before the landing traffic arrives. A standing start from an intersection with a 15 percent margin leaves very little buffer if anything delays the roll.
Judgement. Seventy-five seconds sounds generous but is not, because the read-back and clearance exchange alone can consume 10-15 seconds on a busy frequency. If you accept, you must be ready to move immediately and must not begin the take-off until the clearance is received. If there is any doubt about being airborne and clear before the traffic crosses the threshold, request the full length and let the landing aircraft go first. The safe decision is the one that keeps the landing traffic and your departure separated by a clear, positive margin, not one that relies on everything going perfectly.