AVOIDING VERTICAL AIRSPACE INFRINGEMENTS ....
Acknowledgements: GASCO FLIGHT SAFETY/Spring 2020 (Geoff Weighell – CEO BMAA)
(Ed.Note: In view of the airspace changes at EGLF, this might be useful!)
“Probably the most fundamental point regarding controlled airspace is that, as it is defined in both horizontal and vertical terms, you must think in 3D; essentially it is a cube of airspace, not a horizontal square. Looking at the plan form on a chart, the horizontal extent is usually clear to see, although in some congested areas where airspace boundaries may meet, a very careful study might be required to make absolutely sure that nothing has been missed ....
.... The vertical limits are not so obvious by just looking at the layout on the chart and need some further research. When you locate them, each horizontal depiction will have an upper and lower limit. It may be that the airspace starts at the ground, in which case the lower limit will be shown as SFC (surface) and cannot be flown underneath; alternatively it may be shown as an altitude, for example 1500 feet, or a Flight Level, for example FL50; both of which indicate that there is usually a gap to fly through between the ground and the airspace. I say usually because in high ground areas the gap can be very small!
Reading the chart in three dimensions to understand the vertical limits of the airspace takes a little practice but should have been included in basic ground school training prior to obtaining a licence to fly. I’m sure it was!
Having planned a flight which has the potential to require flight below controlled airspace, it is important that the correct altimeter setting is used at all times to ensure that you are at the level you think you are as indicated by your altimeter. The common altimetry mistakes leading to a vertical infringement are based upon mis-setting the pressure. We all know that the ambient pressure may change over distance and that when travelling any distance the pressure may fall or rise, meaning that the altimeter must be reset otherwise it will over or under read and your aircraft will be higher or lower than you think. The solution is to call an aerodrome which you pass en-route, to update the QNH.
When flying under a CTA or TMA, always set the QNH at an airfield beneath the Area because that is what the base level, when it is an altitude, is determined by. A mistake sometimes made is to continue with a Regional Pressure Setting set when not beneath a CTA or TMA, which may have been determined a long way from where you are and will be the area lowest, usually putting you above the level your altimeter says you are at.
When flying below a Flight Level limit, remember that like investments Flight Levels can go down as well as up with pressure changes. When we get a low-pressure day of say 986 hectopascals, Flight Level 50 will be at 4190 feet above sea level. If you had set QNH and flown at 4500 feet indicated, you would be an infringer! Pressure low = Flight Levels low .... So, having made sure that you know the lower limit of the airspace that you want to fly under by diligent map reading and planning, and being clear on which altimeter setting you are going to use where and keep updated, you should avoid a vertical infringement.
However, one of the most common reasons for vertical infringements is flying so close to the boundary that just a little upset in your handling bumps you up, and hey presto you are an infringer! Published a couple of years ago the Take 2 initiative suggests that you plan to fly no closer than 200 feet vertically to airspace boundary, and two miles horizontally. Obviously, there can be times when you need to be closer, but even so a good margin protects you from small errors which can lead to major consequences.
Finally, when planning to fly beneath controlled airspace make sure if you must descend to get under it that you start your descent well in time, and when climbing having crossed underneath it that you don’t start too early....”.
FLY SAFE!