znightflyer replied to autopilot's topic in Terminal Procedures ForumOne needs to understand why there are SID or OD. A SID is designed to separate aircraft on departure (helps ATC to separate traffic) and a OD are designed to avoid obstacles on the departure (helps pilots avoid hitting something on departure). Sometimes these are combined. Obstacle Departure: An airport that has an instrument approach will be evaluated to see if there are any obstacles that would interfere with a plane climbing at 200' per/NM. If there is an obstacle that penetrates this 200' per/NM climb the FAA will develop a obstacles departure procedure for this airport. Airports with an instrument approach but no OD imply that an aircraft can be assured to climb at 200' per/NM - or 3.3% climb gradient. Standard Instrument Departure are developed to assist ATC in separating aircraft. There may be speed and altitude restraints. Now there are new SID with "climb via SID's". (This is similar to some STAR's)
As far as the 1.23 vs 1.3, it is in the certification of the aircraft under FAR part 25. If I recall it 1.23 is used if the aircraft has a stall pusher. In the real world, Vref is a factor of A/C wt, A/P temp and A/P altitude. Then modified with flap selection and wind factors. Vref corrected by wind normally is never greater than Vref+20.