I forgotten if an alternate is required if you don't have a WAAS GPS regardless of the 1-2-3 rule which states that 1 hour before and after ETA at your destination airport, the ceilings must be at least 2000' and 3 miles visibility. Must we file an alternate or is it dependent on the 1-2-3 rule? Which regulation talks about this because I can't find it.
FAR 91.167 -
(a) No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to -
(1) Complete the flight to the first airport of intended landing;
(2) Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and
(3) Fly after that for 45 minutes at normal cruising speed or, for helicopters, fly after that for 30 minutes at normal cruising speed.
(b) Paragraph (a)(2) of this section does not apply if:
(1) Part 97 of this chapter prescribes a standard instrument approach procedure to, or a special instrument approach procedure has been issued by the Administrator to the operator for, the first airport of intended landing; and
(2) Appropriate weather reports or weather forecasts, or a combination of them, indicate the following:
(i) For aircraft other than helicopters. For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.
This regulation (91.167) applies to civil aircraft regardless of the equipment used for navigation (e.g. GPS/WAAS).
However, if your aircraft has a properly installed and operated GPS (see the TSO requirements in the AIM/AC references below) you can flight plan to your destination and alternate as follows:
TSO-C129() and TSO-C196() equipped (GPS aircraft) - Either the destination or alternate, but not both, can offer only a GPS approach (no other type approaches need be available)
TSO -C145/-C146 (WAAS Equipped Aircraft) - Both the destination and the alternate can be airports with only GPS approaches available.
References: AIM 1-1-17 b 5 c/d, 1-1-18 c 9 a. AND AC-105A, paragraph 9.5.1