Wake turbulence spacing after a heavy departs a short high-elevation runway
At Cochabamba (CBB, elevation 8,360 ft), runway 32 is in use, length 2,650 m, temperature 28 C, QNH 1020. A Boeing 767-300 departs at 15:02, rotates late and is reported airborne abeam the departure end. A Bombardier CRJ-200 (MTOW about 24 t) is holding at the threshold, ready for immediate departure, and a Piper PA-31 is second in line. Approach control also wants to launch a Cessna 210 on a VFR local flight behind the CRJ. The tower controller has a departure gap of about 90 seconds before the next arrival, a Boeing 737 on a 4-mile final.
1. Apply wake turbulence departure spacing. Which of the following may depart, and in what order and with what interval: the CRJ, the PA-31, the Cessna 210? Give the reasoning category by category.
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The 767-300 is a Heavy (MTOW above 136 t). The CRJ-200 is Medium; the PA-31 and Cessna 210 are Light.
CRJ behind 767 (Medium behind Heavy): standard minimum is 2 minutes if departing from the same runway, or 3 minutes if departing from an intersecting runway or a parallel runway less than 760 m apart, or if the preceding aircraft is a Heavy and the following is using a departure path that will cross the Heavy's flight path at the same level. Here, same runway, so 2 minutes applies. The 90-second arrival gap does not override wake separation; the arrival must be sequenced around it or the CRJ must wait.
PA-31 behind 767 (Light behind Heavy): 3 minutes from the same runway if the Light will fly through the Heavy's flight path. If the tower can ensure the PA-31's departure path will not cross the 767's wake-generating flight path, the minimum can be reduced. Because the 767 rotated late and the PA-31 will climb over the same terrain-constrained departure corridor, the safe assumption is that it will cross the wake, so 3 minutes.
Cessna 210 (Light) behind CRJ (Medium): 2 minutes if using the same runway and the Light will cross the Medium's flight path; otherwise the wake minima may be waived if the tower ensures no crossing.
Order: CRJ first (2 min after the 767), then the Cessna 210 or PA-31, each 3 min after the 767 if crossing its path, or 2 min after the CRJ. The practical sequence is 767, CRJ at +2 min, then the light aircraft at +3 min from the 767 (which is also +1 min after the CRJ, so the CRJ-to-Light 2-minute rule governs and pushes the Light to +4 min from the 767). The tower must not compress these intervals to fit the 737 on final; instead it should extend the 737's final or break it off.
2. The tower wants to reduce the interval by having the CRJ depart from the intersection partway down runway 32. What does this change, and what performance and wake considerations apply?
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Intersection departure changes two things.
Wake spacing: the same-runway minima still apply if the intersection is on the same runway and the following aircraft's flight path crosses the Heavy's wake; the reduction is not automatic. What changes is the crossing assessment: an intersection departure may put the CRJ's initial climb-out laterally or vertically clear of the 767's wake if the 767 had already turned, but over a high-elevation airfield with a single departure corridor, that is unlikely to be demonstrable. The controller should not assume a reduction.
Performance: CBB is at 8,360 ft with 28 C. Density altitude is roughly 8,360 + 120 x (28 - ISA temp at that elevation). ISA at 8,360 ft is about 15 - 2 x 8.36 = -1.7 C, so ISA deviation is about +30 C, giving a density altitude near 11,900 ft. A CRJ-200 departing from an intersection loses several hundred metres of runway; at that density altitude its accelerate-stop and single-engine climb gradients become critical. The controller cannot know the CRJ's performance figures, but must accept the pilot's decision and must not pressure for an intersection departure on a hot, high day.
Arrival interaction: a 737 on 4-mile final will be over the threshold in roughly 90 seconds at typical approach speeds. If the CRJ takes the intersection and departs, the tower must confirm the CRJ is airborne and turning clear before the 737 crosses the threshold; otherwise the 737 goes around. Wake spacing and runway occupancy are both binding constraints, and the intersection option does not relieve either.