Under what conditions would a helicopter be allowed to fly at low altitude over populated areas? Yesterday, the Department of Energy was flying a military helicopter at low altitudes over Boston and the surrounding area as part of an experiment of some type to measure radiation.

I saw one of these aircraft when I driving through Watertown and in fact it passed right over me. It was operating at a very dangerous altitude. Any kind of engine failure would have created a multiple fatality situation. Normally, aircraft do not fly at high speed below 500 feet and when helicopters descend below 500 feet they normally do so cautiously and in an area they know to be safe. Having a large helicopter flying at high speed over a dense city at 100 feet AGL is very dangerous and against FAA regulations.

The type of helicopter was not the civvy type pictured in the news story, it was a Chinook. For NASA to be circulating a photo with a little civvy helicopter and then actually flying around a big Chinook I thought was pretty deceptive on their part. 

I have not filed a pilot complaint about this because I assume some kind of special "permission" or exception was created to allow for this dangerous activity. Does anybody know the legal authority involved here and conditions?

BTW, I did not check the NOTAMs and did not get any NOTAMs in my mailbox on this, so if somebody can link the NOTAM that would be useful.



Update: this may have been a medevac flight, with emphasis on the word "may". Apparently, in some cases military helicopters are used for medevac flights to Burlington. However, if it was, the pilot was hot dogging and off the normal routes, both of which are serious regulation violations, but nevertheless I am told some military pilots disregard the rules.
I have no idea about the specific flight you saw, but the general answer to your question about operating legally below the minimum safe altitudes in 14 CFR 91.119 is that the FAA can grant a waiver allowing lower flight. The waiver process says that to get a waiver to 91.119(b) (flight over congested areas) there must be a public interest reason but aircraft can't be lower than 500ft unless lives are at stake:


  When issuing the waiver, the altitude requested should be an absolute
  minimum, but may not be less than 500 feet from persons or property,
  unless necessary to safeguard human life


There isn't enough information about the helicopter you saw to say what was going on but assuming that it really was a Chinook at at 100' AGL then it was too low even for a waiver, according to the FAA's process. But the most obvious explanation is simply that it was taking off; helicopters take off in a range of attitudes and airspeeds and some of those takeoffs are probably relatively low and fast. It might have been a medevac flight (they're often brightly painted) or perhaps something to do with logistics or construction; if you got the N number then you could look it up.