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Sometimes class E airspace goes down to 700 AGL to protect IFR traffic departing/approaching an airport. But I'm failing to understand how this works in practice. Let's look at one example: N51 Solberg airport.

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There's class E going down to 700 AGL surrounding N51. And N51 has a RNAV (GPS) RWY 4 IAP that has MDA of 511 AGL.

Suppose the cloud is at 699 AGL, and some person is flying VFR at 698 AGL (it's class G so they just need to stay clear of clouds), and I'm flying the RNAV RWY 4 approach. As I break out of the clouds with a 500 ft/min descend rate I could immediately collide with that VFR traffic.

So how does this class E airspace offer any safety guarantee to IFR traffic?

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    $\begingroup$ Many other examples exist - e.g. CVO. The short answer to your question (too short to post as an actual answer) is -- "it doesn't." $\endgroup$ Aug 2, 2022 at 17:00

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It is not about VFR. Airspace is about ATC, and ATC is about separating IFR traffic from IFR traffic, and IFR traffic from terrain. Class E doesn't mean radar coverage, it only determines who has this IFR separation responsibility.

What 700ft provides is the ability to vector and set air routes at 1000ft agl, and thus the initial fix for an approach. It also allows much better traffic flow because ATC can't stack class E instrument approaches, it is considered a black box and they can only clear one airplane for the approach at a time.

A 300ft buffer is required from the bottom of the airspace when designing routes or vectoring traffic, 700 becomes 1000agl. (Including a surrounding obstacle-free buffer area and rounded up to an MSL divisible by 100.)

So the short version is that it provides a known obstacle-free area to a lower altitude. Which can also affect the safety of missed approaches and holding.

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    $\begingroup$ Not sure I follow exactly. ATC cannot vector aircraft below the MVA/MIA (with some exceptions) and that altitude will be at least 1000' higher than any terrain or obstructions in the vicinity, that is, it is very very unlikely for the MVA to be any lower than 1300-1500 AGL or so. You are right that ATC doesn't vector in Class G as a general rule, but the E dropping from 1200 AGL down to 700 AGL doesn't help us at all really. $\endgroup$
    – randomhead
    Oct 1, 2022 at 14:18
  • $\begingroup$ @MaxPower - surely the FAA has some guidance on how to adequately protect IFR traffic (e.g. just emerging from clouds) from VFR traffic? I have a hard time believing that these 700' class E floors were not intended to help with this- in some way-- yet at the same time it seems that they are sometimes spectacularly far from adequate-- $\endgroup$ Oct 1, 2022 at 20:13
  • $\begingroup$ Minimum VFR cloud clearance in class E is 152, if it remained class G it would be clear of clouds, so it changes that much. It has been a few years since I was tuned up on the specifics of terminal procedures and airspace design so I will need to come back after refreshing myself. But there are a lot of cases where they just rely on the "big sky" method rather than positive separation. eg in northern Canada an IFR flight is not given ATC separation,(basically IFR in class G) traffic per volume of airspace is so low that the odds of air to air collision are considered insignificant. $\endgroup$
    – Max Power
    Oct 1, 2022 at 22:55
  • $\begingroup$ @randomhead The floor of the airspace must be at least 300 feet below the MVA, if the MVA is 1000agl then the airspace must be 700 or lower. $\endgroup$
    – Max Power
    Nov 12, 2022 at 15:21

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