Mdthbs

Flying VFR around the Bay Area I’ve had two experiences recently that made me a bit nervous, but I haven’t been able to find recommendations that fit the situation.

The first was flying (C172) north along the east bay hills somewhere between OAK and LVK, around 3500’. We saw a 737 descending towards OAK ahead and above us, crossing right to left. 2 or 3 minutes later, long after I’d forgotten about it, I think we crossed the descending wake: we felt a single very sharp bump, hard enough to feel like we’d hit something.

The second time, I was crossing the bay just east of SQL on a northeast heading. ATC pointed out an A380 on final to SFO ahead of us. They passed in front and above us, and I turned to cross their path at 90 degrees, thinking this would be the right strategy for minimizing the risk of being rolled by the vortices. When I later checked the flightradar data, they were about 1000’ above us where our paths crossed. We never felt a bump, though the controller who watched this asked me how the turbulence was, so he evidently thought we were taking a risk.

Of course I know the mantra about wake turbulence is to just avoid it. But in practice, in a situation like this, I might have to divert for 5 minutes or so to try to fly “around” the wake area (e.g., in this case turn SE paralleling the flight path in reverse before turning back on course) and it seemed to me that crossing a wake at 90 degrees seems like a reasonably safe thing to do, a couple of minutes after the plane had crossed (though I couldn’t really judge the vertical separation).

I can’t find anything in faa docs on this situation. They’re all about takeoff and landing. And the bad stories about planes being damaged/destroyed by wakes seem to always involve a near 0 or 180 relative heading (i.e., flying along the vortex line so the two wings see a large force difference).

I’m curious what others think about this.

Due to the nature of wake turbulence descending over time and weakening as time goes on, I would say that it's possible that you hit the wake of the 737, and just managed to avoid the A380's wake out of luck. It also depends on the wind direction as to the path of the wake, so it's hard to say in your specific case

Skybrary describes crossing wake turbulence as:

A cross-track encounter en route is likely to lead to only one or two sharp 'jolts' as the vortices are crossed.

Which leads to me to believe you hit their wake, and not just some other form of turbulence.

In terms of wake avoidance the FAA says in 7.3.7(b).9:

En route VFR (thousand-foot altitude plus 500 feet). Avoid flight below and behind a large aircraft's path. If a larger aircraft is observed above on the same track (meeting or overtaking) adjust your position laterally, preferably upwind.

And skybrary recommends:

In the case of crossing tracks, it has been found that the time interval necessary to avoid creating a wake vortex hazard is 3 minutes.

In terms of a direction to cross the vortexes at this section of FAA 7.3.3(b).1 implies that you should do as you did and cross perpendicular to the path - as to avoid strong induced rolls:

During inflight testing, aircraft intentionally flew directly up trailing vortex cores of larger aircraft. These tests demonstrated that the ability of aircraft to counteract the roll imposed by wake vortex depends primarily on the wingspan and counter-control responsiveness of the encountering aircraft. These tests also demonstrated the difficulty of an aircraft to remain within a wake vortex. The natural tendency is for the circulation to eject aircraft from the vortex.

To summarise those points in terms of enroute wake avoidance, and effect limitation:

• Fly above their path


• Position yourself upwind away from the wake path


• Try to be at least 3 minutes behind the aircraft when crossing the path


• And cross perpendicular to the wake path, to avoid inducing a roll

Sources: https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_3.html
https://www.skybrary.aero/index.php/En-route_Wake_Vortex_Hazard