SOIAs allow simultaneous approaches to two parallel runways spaced at least 750 feet apart, but less than 3,000 feet. The SOIA procedure utilizes an ILS/PRM approach to one runway and an offset Localizer-Type Directional Aid (LDA)/PRM approach with glide slope to the adjacent runway. The use of PRM technology is also required with these operations; therefore, the approach charts will include procedural notes such as “Simultaneous approach authorized with LDA PRM RWY XXX.” San Francisco has the first published SOIA approach. [Figure 5-46]

The training, procedures, and system requirements for SOIA ILS/PRM and LDA/PRM approaches are identical with those used for simultaneous close parallel ILS/PRM approaches until near the LDA/PRM approach MAP, except where visual acquisition of the ILS aircraft by the LDA aircraft must be accomplished. If visual acquisition is not accomplished a missed approach must be executed. A visual segment for the LDA/PRM approach is established between the LDA MAP and the runway threshold. Aircraft transition in visual conditions from the LDA course, beginning at the LDA MAP, to align with the runway and can be stabilized by 500 feet above ground level (AGL) on the extended runway centerline.

The FAA website has additional information about PRM and SOIA, including instructional videos at:


Another method by which ILS approach capacity can be increased is through the use of converging approaches. Converging approaches may be established at airports that have runways with an angle between 15 and 100 degrees and each runway must have an ILS. Additionally, separate procedures must be established for each approach and each approach must have a MAP at least 3 NM apart with no overlapping of the protected missed approach airspace. Only straightin approaches are approved for converging ILS procedures. If the runways intersect, the controller must be able to visually separate intersecting runway traffic. Approaches to intersecting runways also have higher minimums with a 700-foot minimum and no less than 2 SM visibility. Pilots are informed of the use of converging ILS approaches by the controller upon initial contact or through ATIS. [Figure 5-47 on page 5-58]

Dallas/Fort Worth International airport is one of the few airports that makes use of converging ILS approaches because its runway configuration has multiple parallel runways and two offset runways. [Figure 5-48 on page 5-58]The approach chart title indicates the use of converging approaches and the notes section highlights other runways that are authorized for converging approach procedures.


The MLS is a precision instrument approach alternative to the ILS. It provides azimuth, elevation, and distance information, as well as a back azimuth capable of providing guidance for missed approach procedures and departures. In addition to straight-in approaches, the MLS system can also provide three-dimensional RNAV type approaches in both computed straight and curved paths. It was initially designed to replace the ILS system and it provided inherent flexibility and broader reception range with the greatest limitation being the capabilities of the airborne equipment installed in individual aircraft.

The MLS has multiple advantages including an increased number of frequencies, compact ground equipment, and complex approach paths. For a variety of reasons, particularly the advent of civil use GPS, MLS installation was deferred, and by 1994 it was officially cancelled by the FAA. Today there are few MLS installations in the U.S. and currently there are no plans for further installations. Futhermore, the MLS equipment required for an MLS approach was not widely installed in aircraft, whereas most new aircraft produced today come with GPS systems. With the limited number of MLS installations around the country, it is highly unlikely that most pilots will ever encounter the MLS approach, and if they do, it is even less likely that the proper equipment would be installed in the aircraft.

Like the ILS, the basic MLS approach requires the final approach course alignment to be within 3 degrees of the extended runway centerline. This type of approach uses a glide slope between 3 and 6.40 degrees and provides precision landing minimums to 200 feet HAT. Obstacle clearance is based on the glide slope angle used in the approach design. The design criteria differ for each type of MLS approach and incorporate numerous formulas for the derivation of specific course criteria. This information is contained in FAA Order 8260.3 Volume 3, Chapters 2 and 3.

In the front of the TPP, there is a page containing additional information pertaining to the use of an MLS system. The MLS Channeling and Frequency Pairing Table cross references the appropriate MLS channel with its paired VHF and TACAN frequencies. Ground equipment associated with the MLS operates on the MLS channels, while the MLS angle/data and DME is required to operate using one of the paired VHF or TACAN frequencies.

Figure 5-46. Simultaneous Offset Instrument Approach Procedure.

Figure 5-47. Converging Approach Criteria.

Figure 5-48. Dallas/Fort Worth (KDFW), Dallas/Fort Worth,Texas, CONVERGING ILS RWY 35C.