DA—a specified altitude in the precision approach at  which a missed approach must be initiated if the required  visual reference to continue the approach has not been  established.  with limited experience in the aircraft they are operating,  increase the approach minimums and visibility by 100  feet and one-half mile respectively. Rules for these “high­ minimums” pilots are usually derived from a combination  of federal regulations and the company’s OpSpecs. There  are many factors that can determine the actual minimums  that can be used for a specific approach. All of them must  be considered by pilots during the preflight and approach  planning phases, discussed, and briefed appropriately.  DH—with respect to the operation of aircraft, means the  height at which a decision must be made during an ILS, MLS,  or PAR IAP to either continue the approach or to execute a  missed approach.  CAT II and III approach DHs are referenced to AGL and  measured with a radio altimeter.  Pilots are cautioned to fully understand and abide by the  guidelines set forth in 14 CFR § 91.175(c) regarding proper  identification of the runway and runway environment when  electing to continue any approach beyond the published  DA/DH or MDA.  The height above touchdown (HAT) for a CAT I precision  approach is normally 200 feet above touchdown zone  elevation (TDZE). When a HAT of 250 feet or higher is  published, it may be the result of the signal-in-space  coverage, or there may be penetrations of either the final  or missed approach obstacle clearance surfaces (OCSs).  If there are OCS penetrations, the pilot has no indication  on the approach chart where the obstacles are located. It  is important for pilots to brief the MDA, DA, or DH so that  there is no ambiguity as to what minimums are being used.  These altitudes can be restricted by many factors. Approach  category, inoperative equipment in the aircraft or on the  ground, crew qualifications, and company authorizations  are all examples of issues that may limit or change the  height of a published MDA, DA, or DH.  It is imperative to recognize that any delay in making a  decision to execute the Missed Approach Procedure at  the DA/DH or MDA/Missed Approach Point will put the  aircrew at risk of impacting any obstructions that may be  penetrating the visual obstacle clearance surface  The visual segment of an IAP begins at DA or MDA and  continues to the runway. There are two means of operating  in the visual segment, one is by using natural vision under  14 CFR Part 91, § 91.175 (c) and the other is by using an  Enhanced Flight Vision System under 14 CFR Part 91, §  91.176.  For many air carriers, OpSpecs may be the limiting factor  for some types of approaches. NDB and circling approaches  are two common examples where the OpSpecs minimum  listed altitudes may be more restrictive than the published  minimums. Many Part 121 and 135 operators are restricted  from conducting circling approaches below 1,000 feet  MDA and 3 SM visibility by Part C of their OpSpecs,  and many have specific visibility criteria listed for NDB  approaches that exceed visibilities published for the  approach (commonly 2 SM). In these cases, flight crews  must determine which is the more restrictive of the two  and comply with those minimums.  In some cases, flight crew qualifications can be the limiting  factor for the MDA, DA, or DH for an instrument approach.  There are many CAT II and III approach procedures  authorized at airports throughout the United States, but  RNP AR restricts their use to pilots who have received  specific training, and aircraft that are equipped and  authorized to conduct those approaches. Other rules  pertaining to flight crew qualifications can also determine  the lowest usable MDA, DA, or DH for a specific approach.  14 CFR Part 121, § 121.652, 14 CFR Part 125, § 125.379,  and 14 CFR Part 135, § 135.225 require that some PICs,  An Enhanced Flight Vision System (EFVS) is an installed  aircraft system which uses a head up display (HUD), or an  equivalent display that is a head up presentation, to combine  aircraft flight information and flight symbology, navigation  guidance, and a real-time image of the external scene to  the pilot on a single display. Imaging sensors, which may  be based on forward-looking infrared (FLIR), millimeter  wave radiometry, millimeter wave radar, low-level light  intensification, or other real-time imaging technologies,  produce a real-time image of the outside scene. Combining  Enhanced Flight Vision Systems (EFVS) and  Instrument Approaches  [Figure 4-10A]  Figure4-10A.View during an approach with EFVS (left) and  without EFVS (right). (Images courtesy of NASA Langley  Research Center) 4-20