Do I need an Emergency Locator Transmitter (ELT) in my homebuilt?
Yes, if you are building an airplane that has more than one seat, you will be required to have an ELT. The regulation that pertains to ELT's is 14 CFR 91.207. This section applies to all US registered civil airplanes, whether they are standard or experimental category.
The specific paragraph in 91.207 that applies to homebuilt aircraft reads as follows:
(a) Except as provided in paragraphs (e) and (f) of this section, no person may operate a U.S.-registered civil airplane unless --
(2) For operations other than those specified in paragraph (a)(1) of this section, there must be attached to the airplane an approved personal type or an approved automatic type emergency locator transmitter that is in operable condition, except that after June 21, 1995, an emergency locator transmitter that meets the requirements of TSO-C91 may not be used for new installations.
Paragraph (a)(1) of 91.207 talks about charter and air carrier operations, and does not apply to the operation of homebuilt aircraft.
Single seat airplanes are exempt from the requirement for an ELT. This exemption is found in 91.207(f)(9), the pertinent part of which is quoted here:
"(f) Paragraph (a) of this section does not apply to --
(9) Aircraft equipped to carry not more than one person."
Thus, a single-seat airplane is not required to have an ELT installed, regardless of certification category.
Note that this regulation speaks specifically to “airplanes”, which the FAA defines as:
“…an engine-driven fixed-wing aircraft heavier than air, that is supported in flight by the dynamic reaction of the air against its wings.”
This being the case, homebuilts other than airplanes (i.e., helicopters, gyroplanes, balloons, gliders, airships, trikes, powered parachutes, etc.) do not require an ELT under this regulation.