When is it safe to land a Cessna-172 after a commercial airline jet has landed?
It depends on the size of the preceding aircraft. ICAO PANS-ATM-Doc 4444 Air Traffic Management has some values.
5.8.2 · Arriving aircraft
22.214.171.124 Except as provided for in 5.8.l.l a) and b), the following separation minima shall be applied.
5.8.2.l.l The following minima shall be applied to aircraft landing behind a HEAVY or a MEDIUM aircraft:
a) MEDIUM aircraft behind HEAVY aircraft- 2 minutes;
b) LIGHT aircraft behind a HEAVY or MEDIUM aircraft- 3 minutes.
Note that in case of A380, it is more- 4 minutes.
In case of distances, the light aircraft are to be landed 5nm after a medium aircraft, 6nm after a heavy and 8nm after A380.
Note: The categorisation of aircraft is done as below:
4.9.1 Wake turbulence categories of aircraft
126.96.36.199 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certificated take-off mass as follows:
a) HEAVY (H)- all aircraft types of 136 000 kg or more;
b) MEDIUM (M)- aircraft types less than 136 000 kg but more than 7 000 kg; and
c) LIGHT (L)- aircraft types of 7 000 kg or less.
The ATC rules in FAA-land say that if a "small" aircraft (maximum takeoff weight up to 12,500 lbs) is operating under instrument flight rules behind a "super" aircraft (A380 or An225) and less than 1000 feet below it, the minimum radar separation is eight miles between the super ahead and the small behind. This is always the case, including when both are on an instrument approach (and therefore the small cannot maneuver visually to avoid wake turbulence). 7110.65 5–5–4 g1(a)(3).
When radar separation is not being used, i.e. when the small is VFR and not being vectored for an approach, there is no minimum separation outside of Class B airspace, where the standard 1.5 NM separation applies. (Not that it's likely an A380 would be landing at a non-Class B airport, but the An225 might!) But the ATC should without a doubt issue a wake turbulence advisory in accordance with 2–1–20 and 3–10–3 b, and should expect the small to maneuver very far behind the super.
As for the actual landing, there are a few different situations, described in 3–10 and 3–9:
- Small is landing behind landing or departing super, same runway. No increased separation beyond standard same-runway separation (lead aircraft must be clear of runway if landing, or 6000 feet down the runway and airborne if departing).
- Small is landing behind landing super, crossing runway. No increased separation beyond standard crossing-runway separation (lead aircraft must be through the intersection).
- Small is landing behind departing super, crossing runway, super will rotate before the intersection. 3 minutes separation.
- Small is departing behind departing super, same runway. 3 minutes or 8 miles on radar.
- Small is departing at an intersection (or touch-and-go) behind departing super, same runway. 4 minutes after super rotates.
- Small is departing behind landing or departing super, crossing runway, super is airborne at the intersection. 3 minutes.
There are a few more cases involving converging runways, or where two arrivals will cross paths prior to the runway thresholds, that are similar to the crossing-runways cases.