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.

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    $\begingroup$ Any kind of engine failure would have created a multiple fatality situation. I believe that this is the flawed premise that negates the entire question. For a Chinook, an engine failure in high speed forward flight is fully controllable - indeed, the type will not gain certification if it is not. Forgive me, but the entire post reads more like a rant than a serious enquiry. $\endgroup$
    – Simon
    Commented Apr 11, 2016 at 19:45
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    $\begingroup$ CFR 91.119(d) "A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA" Also see Boston Helicopter Routes (Click on the Helicopters tab). You are also 2 days early, and they say they are flying a grid pattern, probably approved by the FAA. $\endgroup$
    – Ron Beyer
    Commented Apr 11, 2016 at 19:47
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    $\begingroup$ I note that the announcement (dated April 11) explicitly talks about a "twin-engine Bell 412" operating "April 12 through April 15". How sure are you that the different helicopter you saw on April 10 is actually anything to do with the NNSA program? The logical assumption would be that they're unconnected... $\endgroup$ Commented Apr 11, 2016 at 19:51
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    $\begingroup$ I have been in a Chinook, at about 50 feet, and around 70kts, with a simulated engine failure. Have you? It does not lose height rapidly. The recovery in this case (flying through a fire break in a forest) was to climb to a more normal altitude so that an auto was possible if the second engine quits. $\endgroup$
    – Simon
    Commented Apr 11, 2016 at 20:08
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    $\begingroup$ How do you know it was 100ft? Chinooks are big, you could easily have misjudged its altitude. $\endgroup$
    – GdD
    Commented Apr 12, 2016 at 11:53

3 Answers 3


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.

  • $\begingroup$ This was my general impression. Going below 500 feet is a no-no, so I am trying to figure out how it is, that this is happening. The chopper I saw was no medevac flight. $\endgroup$ Commented Apr 11, 2016 at 21:35
  • $\begingroup$ @TylerDurden Why do you say that going below 500 feet is a no-no? On what basis? We are talking about helicopter operations here, aren't we? And military helicopter operations, what is more. I am curious to know what I am missing regarding why low level flight would be prohibited. $\endgroup$
    – J W
    Commented Nov 19, 2016 at 17:44

By your description a "military" Chinook-type helicopter was flying at high speed over a congested area at about 100 ft AGL.

The answer to your title question—Under what conditions would a helicopter be allowed to fly at low altitude over populated areas?—is twofold.

  1. Firstly, under FAA regulations (more on this in a moment), helicopters are not generally subject to any specific altitude restrictions. See 14 CFR 91.119 Minimum safe altitudes: General.

    Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:

    (a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.

    (b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

    (c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

    (d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—

    (1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA

    The FAA comments on this regulation:

    Helicopter operations may be conducted below the minimum altitudes set for fixed-wing aircraft. The reason? The helicopter's unique operating characteristics, the most important of which is its ability to execute pinpoint emergency landings during power failure. Further, the helicopter's increased use by law enforcement and emergency medical service agencies requires added flexibility in the application of many FAA provisions.

    Therefore, a helicopter may be operated at an altitude which complies with §91.119 (a) and (d), despite perhaps appearing to be otherwise to the unknowing observer.

  2. Secondly, military aircraft operations are generally not subject to the same FAA regulations that civilian aircraft operations are. Therefore a military helicopter operation, such as that which you describe, might well be operated under military aviation rules in disregard to rules that would otherwise govern civilian helicopter operations. See this answer regarding the FAA's jurisdiction—or lack thereof—over military aircraft operations.

Taken together, we see that the military helicopter operation that you describe might well have been operated in compliance with 14 CFR 91.119—and according to Simon's comment above, it probably was; we also see, however, that the military will operate according to it's own rules. The answer to the question in the body of your text, Does anybody know the legal authority involved here and conditions?, can best be answered: "by the authority of the US Government, under whatever conditions they deemed appropriate".


The quick, short answer - It depends.

Remember, there are two distinct sets of rules involved here.

  1. The FARs, they apply to the vast majority of aircraft (Fixed wing and Rotary wing).

  2. Public use aircraft - military, local government aircraft and pilots. These aircraft and pilots are not necessarily operating under the control/regulation of the FAA. Just because the City, County or State entities require the pilots to be licensed doesn't mean they must do this.


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