Joined: 11 Dec 2005
|Posted: Thu Feb 21, 2008 4:49 pm Post subject: 737-300 Non-ILS Instrument Approaches
| During Non-ILS approaches, autopilot use allows better course and vertical path tracking accuracy. It reduces the probablity of inadvertent deviations below path. Curreltly, the VNAV PTH mode contains no path deviation alerting. For this reason, the autopilot should remain engaged until suitable visual reference has been established.
RNAV approaches may be flown with any RNP alerting capable FMC. FMCs that are not RNP alerting capable may be used for RNAV approaches provided the following requirements are met:
- The approach has an RNP no smaller than 0.5NM
- The pilot ensures that DME-DME updating is active before commencing the approach
- V/S is used on the final approach
To use LNAV for approaches and missed approaches, a proper series of legs/waypoints that describe the approach route (and missed approach) must appear on the LEGS page. No waypoints may be added or delted between the FAF and the MAP. If the approach to be flown is not in the database, another approach having the same plan view may be selected. For exmaple, an ILS approach might be selected if the plan view (route) is identical to an NDB approach. In this case, waypoint altitudes must be checked and modified as required. When an approach is flown by this "overlay" method, raw data should be monitored throughout the approach to assure obstacle clearance. If an NDB approach for the desired runway is in the database, an overlay approach should not be used.
If a waypoint is added or deleted from a database procedure, FMC "on approach" logic is partially or completely disabled and the VNAV obstacle clearance integrity of the procedure may be adversely affected. If an additional waypoint reference is desired, use the FIX page and do not modify waypoints on the LEGS page.
Pilots should not become involved in excessive "heads down" FMC manipulation to build MAP displays while at low altitude. Raw data VOR, ILS and ADF displays should be used to avoid distractions during higher workload phases of flight. Map building should be avoided below 10,000 feet AGL.
VNAV should be used only for approaches that have one of the following features:
- A published GP angle on the LEGS page for the final approach segment
- An RWxx waypoint coincident with the approach end of the runway
- A missed approach waypoint before the approach end of the runway (e.g. MXxx)
When appropriate, crew should make cold temperature altitude corrections by applying a correction from an approved table to the waypoint altitude constraints. The FMC obtains the GP angle displayed on the LEGS page from the navigation database. This GP angle is based on the standard atmosphere and is used by the FMC to calculate the VNAV path which is flown using a barometric reference. When OAT is lower than standard, true altitudes are lower than indicated altitudes. Therefore, if cold temperature altitude corrections are not made, the effective GP angle, is lower than the value displayed on the LEGS page. When cold temperature altitude corrections are made, VNAV PTH operation and procedure tuning function normally; however, the airplane follows the higher of the glide path angle associated with the approach (if avaialble) or the geometric path defined by the waypoint altitude constraints. Temperature corrections redefine the glide path only if the FMC has the geometric path option installed.