Aerobatic laws in the USA
 

Here's one for the US FAA gurus........
Here is the extract from FARs about aerobatic flight in the USA.

§ 91.303 Aerobatic flight.
No person may operate an aircraft in aerobatic flight -

(a) Over any congested area of a city, town, or settlement;

(b) Over an open air assembly of persons;

(c) Within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport;

(d) Within 4 nautical miles of the center line of any Federal airway;

(e) Below an altitude of 1,500 feet above the surface; or

(f) When flight visibility is less than 3 statute miles.

Item (d) (Within 4 nautical miles of the center line of any Federal airway) is the query.
Is it permitted to carry out aerobatics beneath and clear of the base of an airway in the USA? The "within 4 miles of the centre line" implies that even if you are in the "free airspace" underneath the airway, you may not carry out aeros because you are within 4 miles of the centreline (although you are not inside the airway).....
In the UK, provided that you remain clear of the airway, you are perfectly at liberty to perform aeros beneath.
I would appreciate a definitive answer from a Qualified Guru (please no "I believe", or "I am fairly sure" etc!!).
Many thanks,
Jez.

As huge a country as ours is, I'm puzzled why you'd be so hellbent on doing aerobatics specifically with 4nm of an airway.

Then again, I'm no guru.

Well, a Victor airway starts at 1,200' of the surface to 17,999' above the surface, so, it would low altitude acro in any case. BUT, the rule says "4 nm either side, so it doesn't matter anyway. We have plenty of airspace off airways.

GF

There is at least one other way to fly aerobatics within the confines of a federal airway that I know of. I believe it falls under the FAA waiver process. A designated aerobatic
"box" may be established in airspace where it is found not to be an unusual hazard. I did my basic aerobatic training at Santa Paula, CA. (KSZP) The SZP "box" was located 5 NM E. of the airport and right under a Victor airway. It went from 2,500 MSL to 5,000 MSL in an area where the ground level is about 300' elevation. To use the aerobatic box required notifying flight service in advance, who would then issue a NOTAM.

If you look down while flying the Fernando arrival to Burbank and passing over the FIM VOR at 11,000', you can sometimes see an airplane 1 NM below, maneuvering in the box.

This kind of arrangement may be less necessary in the sparsely populated areas of flyover country than it is in more crowded airspace such as that found in or near the Los Angeles or other metropolitan areas. Here, such waiver "boxes" are somewhat more common. One would have to fly a long ways to get clear of airways otherwise.

I'm puzzled why you'd be so hellbent on doing aerobatics etc
 

Amadis, to save you being puzzled............
I am privileged to fly a collection of aircraft belonging to a museum in the USA based at an airfield that is surrounded as far as the eye can see by 8/8 large trees. For obvious reasons, we always fly these priceless aircraft within gliding distance of the airfield since it is the only suitable landing spot within a 10 mile radius. Some of these aircraft are aerobatic and need to be exercised!
I was merrily cavorting around in the Stearman about a mile away from the airfield and upon landing a nice gent wondered over and kindly pointed out that what I was doing was super to watch, but was illegal because of the proximity of an airway. I respectfully pointed out that the airway base was 4500' and I was well below that. He continued to educate me as to the error of my ways. I have of course, desisted forthwith, but it has always vexed me that this cannot be true.
Thank you guys for pointing out that it is indeed true (strange....but true!).
Jez.

Did this gentleman have a badge or was this just someone educating you with his own priceless opinions?

Quite an unusual situation you describe there...

His priceless opinions.........No badge but he was a much respected (by me) experienced Flight Instructor and Airline Captain.
I'm still not sure though, but who am I (a simple Limey) to try and understand?
Which is why I asked for clarification from the knowledgable people here.
FARs don't really help, they appear extremely ambiguous and rather "woolly".
Hey ho,
Thanks guys,
Jez

I feel your pain there. I would have just bowed and declared myself desperately unworthy.

just bowed and declared myself desperately unworthy
 

That's more or less what I did do....... Then climbed into a stunning original Pitcairn Super Mailwing and exercised that (non-aerobatically!)...............
Life is full of compensations :-)
Jez