feet MSL) approximately 1.3 NM from the threshold and in  a position to land within the TDZ. Techniques for deriving a  300-to-1 glide path include using DME, distance advisories  provided by radar-equipped control towers, RNAV, GPS, dead reckoning, and pilotage when familiar features on the  approach course are visible. The runway threshold should be crossed at a nominal height of 50 feet above the TDZE. The transition from instrument flight to visual flight during  an instrument approach can be very challenging, especially  during low visibility operations. Aircrews should use caution  when transitioning to a visual approach at times of shallow  fog. Adequate visibility may not exist to allow flaring of  the aircraft. Aircrews must always be prepared to execute  a missed approach/go-around. Additionally, single-pilot  operations make the transition even more challenging.  Approaches with vertical guidance add to the safety of  the transition to visual because the approach is already  stabilized upon visually acquiring the required references  for the runway. 100 to 200 feet prior to reaching the DA,  DH, or MDA, most of the PM’s attention should be outside  of the aircraft in order to visually acquire at least one visual  reference for the runway, as required by the regulations.  The PF should stay focused on the instruments until the  PM calls out any visual aids that can be seen, or states  “runway in sight.”The PF should then begin the transition  to visual flight. It is common practice for the PM to call out  the V/S during the transition to confirm to the PF that the  instruments are being monitored, thus allowing more of  the PF’s attention to be focused on the visual portion of the  approach and landing. Any deviations from the stabilized  approach criteria should also be announced by the PM.  Single-pilot operations can be much more challenging  because the pilot must continue to fly by the instruments  while attempting to acquire a visual reference for the  runway. While it is important for both pilots of a two-pilot  aircraft to divide their attention between the instruments  and visual references, it is even more critical for the  single- pilot operation. The flight visibility must also be  at least the visibility minimum stated on the instrument  approach chart, or as required by regulations. CAT II and III  approaches have specific requirements that may differ from  CAT I precision or non-precision approach requirements  regarding transition to visual and landing. This information  can be found in the operator’s OpSpecs or FOM.  The visibility published on an approach chart is dependent  on many variables, including the height above touchdown  for straight-in approaches or height above airport elevation  for circling approaches. Other factors include the approach  light system coverage, and type of approach procedure,  4-39 Many reasons exist for executing a missed approach. The  primary reasons, of course, are that the required flight  visibility prescribed in the IAP being used does not exist  when natural vision is used under 14 CFR Part 91, § 91.175c,  the required enhanced flight visibility is less  than that  prescribed in the IAP when an EFVS is used under 14 CFR  Part 91, § 91.176, or the required visual references for the  runway cannot be seen upon arrival at the DA, DH, or  MAP. In addition, according to 14 CFR Part 91, the aircraft  must continuously be in a position from which a descent  to a landing on the intended runway can be made at a  such as precision, non-precision, circling or straight-in.  Another factor determining the minimum visibility is the  penetration of the 34:1 and 20:1 surfaces. These surfaces  are inclined planes that begin 200 feet out from the runway  and extend outward to the DA point (for approaches with  vertical guidance), the VDP location (for non-precision  approaches) and 10,000 feet for an evaluation to a circling  runway. If there is a penetration of the 34:1 surface, the  published visibility can be no lower than three-fourths SM.  If there is penetration of the 20:1 surface, the published  visibility can be no lower than 1 SM with a note prohibiting  approaches to the affected runway at night (both straight- in and circling). [Figure 4-21 ] Circling may be permitted  at night if penetrating obstacles are marked and lighted.  If the penetrating obstacles are not marked and lighted,  a note is published that night circling is “Not Authorized.”  Pilots should be aware of these penetrating obstacles  when entering the visual and/or circling segments of an  approach and take adequate precautions to avoid them.  For RNAV approaches only, the presence of a grey shaded  line from the MDA to the runway symbol in the profile view  is an indication that the visual segment below the MDA is  clear of obstructions on the 34:1 slope. Absence of the gray  shaded area indicates the 34:1 OCS is not free of obstructions.  [Figure 4-22]  Missed Approach  Transition to a Visual Approach  Figure 4-21. Determination of visibility minimums.  27  Runway  Runway  200 feet  Not to scale  20:1 34:1 10,000 feet 