Crop duster aircraft operate much differently than most other aircraft. Are there special rules that apply to their operation? Specifically, are there rules about how close they can fly to structures, terrain, or vehicles?
14 CFR Part 137 regulates agricultural aircraft operations, with §137.51 discussing operation over congested areas, including:
The crop duster must operate “with maximum safety to persons and property on the surface” and the crop duster must have obtained prior written approval from the FAA and notice of the intended operation must be given to the public. Further, the crop duster must submit a plan to the FAA that considers the flight obstructions and plans for a possible emergency landing.
Of utmost importance is the section regarding the flying of restricted category civil aircraft such as agricultural aircraft:
Sec. 91.313 — Restricted category civil aircraft: Operating limitations.
(a) No person may operate a restricted category civil aircraft— (1) For other than the special purpose for which it is certificated; or
(2) In an operation other than one necessary to accomplish the work activity directly associated with that special purpose.
(e) Except when operating in accordance with the terms and conditions of a certificate of waiver or special operating limitations issued by the Administrator, no person may operate a restricted category civil aircraft within the United States—
(1) Over a densely populated area;
(2) In a congested airway; or
(3) Near a busy airport where passenger transport operations are conducted.
More information is available in this FAA document.
Ag applicators, crop dusters, are generally governed by 14CFR91 for the operation of their aircraft, except when actually performing ag operations, which includes application and travel to and from loads and applications, they are governed by 14CFR137.
Operations over other than congested areas, 14CFR137.49 permits operations below 500 ft AGL, and closer than 500 feet to persons, vessels, vehicles, and structures.
Operations over congested areas, 14CFR137.51 permits wider exceptions to the general rules under 14CFR91, but with various provisions for helicopters, single engine and multi engine airplanes. Additionally, it makes clear that minimum altitude deviations from 14CFR91 are limited to dispensing and approaches and departures for dispensing. It does not relieve the operator from flying at an altitude which provides safety in the event of an emergency, without endangering persons or property on the surface.
Furthermore, 14CFR137.53 establishes requirements for pilot experience and aircraft inspections when ag aircraft are operated over congested areas.
The technology for ag aircraft is changing, and while recip Otto cycle aircooled engines have been employed traditionally, it is much more common to utilize turboprop powerplants. Additionally, turboprop conversions are popular for some traditional ag aircraft. The turboprop enhances reliability substantially, and are equipped with inertial separators to reduce the likelihood of things like the ingestion of birds from causing powerplant failures. Additionally turboprops have enhanced higher altitude performance which traditionally called for the use of more complex turbocharged recip engines or multi engine ag aircraft.
In general, ag pilots have substantial experience, and in general are well versed at low altitude operations. There are some academic and also flight school operations which specialize in training ag pilots.
In short, with appropriate planning, and a good working relationship with the local FAA FSDO office, and appropriate public relations (notice to public required for some operations over congested areas under 14CFR137.51) ag applicators are given wide latitude to perform their function while assuring acceptable levels of public safety.