I have heard about Simultaneous Offset Instrument Approaches (SOIA), special procedures for runways that are spaced particularly close together. How exactly do they work? What are the requirements for an airport to be eligible for the procedure? What special requirements are there for pilots flying the approach?
SOIA (Simultaneous Offset Instrument Approaches) allow airports with parallel runways that are 750 to 3000 feet apart to conduct (almost) simultaneous approaches to the two runways. At an airport, one runway uses the ILS PRM approach, while the other runway uses an offset LDA PRM approach (with glideslope). SOIA refers to the LDA PRM approach, where another aircraft is flying the ILS PRM approach ahead. PRM stands for Precision Runway Monitor, which is a high-update radar system allowing for "equivalent" safety with reduced margins.
For a PRM approach is conducted, there must be two frequencies per runway:
- a primary frequency for both the transmission and receipt of instructions
- a "monitor" frequency, only for ATC to transmit on
The reason for the "monitor" frequency is just in case the primary frequency is blocked and breakout instructions have to be provided. The tower controller will communicate to aircraft on both frequencies, both in normal PRM operations and when providing breakout instructions.
There is also a "monitor" controller that ensures that the two aircraft never get dangerously close together -- if such a situation occurs, the "monitor" controller overrides the tower controller on both frequencies to allow for immediate breakout instructions to be given.
Of the two approaches, the more interesting of the two is the offset LDA PRM approach. In the LDA PRM approach, aircraft fly an approach course that is 2.5 to 3 degrees offset from the runway heading, following the glideslope until the MAP (this course does not meet up with the runway threshold), at which point the pilot should proceed visually to the runway.
The MAP on a LDA PRM approach is 3000 feet away from the ILS approach course, meaning aircraft have to manoeuvre towards the extended runway centreline to be stable by 500 feet -- the less spacing between the two runways, the more aircraft will have to turn. This does mean that it is less suitable to larger aircraft, due to the manoeuvring required at higher speeds (than other aircraft), though most SOIA approaches are designed for larger aircraft as well. Before reaching the MAP, the pilot must have have:
- the runway in sight;
- the aircraft on the adjacent ILS PRM approach in sight; and
- notified ATC that traffic is in sight.
This means that the aircraft on the ILS PRM approach should be ahead of the other aircraft. To allow time for visual contact, aircraft should be clear of cloud before the MAP, which is taken into account when deciding on the minimum cloud ceiling allowed for the approach.
The NTZ (No Transgression Zone) is a 2000 foot wide area between the two approach courses which aircraft must not enter at any time. ATC would immediately issue breakout instructions to both aircraft if an aircraft breaches the NTZ, as there would be very little horizontal separation between the two aircraft. [The FAA AIM] states that:
In the unlikely event that an aircraft "blunders" off its course and makes a worst case turn of 30 degrees toward the adjacent final approach course, closing speeds of 135 feet per second could occur that constitute the need for quick reaction.
Special training requirements vary from country to country. In the US, SOIA special training requirements are:
- Parts 121/129/135: operator training as agreed to by FAA and educational video
- Part 91 transport category: read about PRM approaches in AIM, video
- Part 91 other: read about PRM and SOIA approaches in AIM, video
An example of a SOIA approach is the SFO runway 28R LDA PRM approach: