>>>"i think it's a specific height such as 1,000ft or something when a decision is made.
did a pretty good job explaining what his company Pilot Operating Manual allows. Usually company POMs will delineate their own procedures--which are in compliance (only stricter) with published procedures. It is important to understand Missed Approach Point, Minimum Descent Altitude, and Decision Height. Each varies according to avionics/flight instruments (POMs will also determine what a high mins Capt can/cannot do), airports, and navaids available...I'll add my 2 centavos.
1. A pilot making an instrument approach (Precision or Non-Precision) will have either a JEPP or IAP chart which will explain the altitudes, flight paths, and weather minimums for the runway at a given airport. A pilot adhering to the published procedures and/or vectors/altitudes assigned by ATC will be assured of terrain clearance and runway alignment.
Approach plates will have:
(a). A visual depiction called the plan view along with appropriate frequencies.
(b). A profile view which depicts minimum altitudes, maximum distance for procedure turn execution, and missed approach procedures.
(c). Minimums section. The minimums section will provide a minimum descent altitude (based on speed 1.3 * VSo) given the avionics available or circling procedure. (On an ILS approach, localizer only will demand higher mins, as too will circling).
2. Under certain conditions, pilots may request (or be assigned) visual approaches. That is, if the following are met:
(a). VIZ 3 miles at the surface. 500ft ceiling above min vectoring alt.
(b). Pilot has a visual tally on airport (or aircraft he is following).
(c). Pilot remains in VMC.
3. Closely related to the visual approach is the contact approach. (Digressing here a little...reading the NTSB report and CVR recordings of the BIZ jet that crashed in Aspen a couple years ago, there is discussion among the crew of executing a contact approach). The pilot requests this (it is not assigned by ATC).
(a). Aircraft clear of clouds.
(b). 1 mile viz
(c). Pilot can reasonably continue in present conditions to airport safely.
(d). Controllers may grant approval to contact approach if the reported viz is 1 mile, airport has an IAP/JEPP, and aircraft separation is maintained.
4. Precision Approach Radars (near and dear to my heart) allow controllers to give verbal glide path, azimuth, and range information to pilots. Basically a PAR will give headings, when to commence descent to MDA, aircraft's position each mile from the runway and arrival at missed approach point.
5. When executing a missed approach, the pilot flies to the missed approach point (at or above MDA or DH
). If the pilot loses visual reference while circling to land, the turn is continued until established on the missed approach course.