Reference Documentation and Cold Weather Ops
The reference document for the construction and operational use of BARO-VNAV and temperature limits is: ICAO Doc 8168. For those of you who are using Jepp FD or Jepp FD Pro, this document is available in the manual list. The manuals located inside should be read. There is a wealth of information contained therein.
Jepp. is NOT regulatory material, it so happens that they produce material based on what regulatory and SARPS promulgate.
I've quoted most of the information here directly from Doc 8168.
In essence, when the procedure designers build an approach for BARO/VNAV, limited temperature effects are accounted for on the descent segment. The temperature limits are there to protect the shallow gradients during a managed or IAN type approach. That said however; the DH/DA/MDAs still must be accounted for with the correct non-standard temperature corrections found in the tables. If the temperature is outside of the published limits then the procedure can still be flown but not managed or IAN and all the altitudes in the descent must be corrected for temperature error.
Some aircraft equipment are certified to calculate their own corrections from the FMS to certain limits. If that is the case, then other corrections can be disregarded as long as the temperatures being flown are within the certification limits of the installed equipment.
This is directly from ICAO Doc: 8168
1.2.2 Atmospheric effects
1.2.2.1 Atmospheric errors associated with non-standard temperatures are considered in the design of the approach obstacle clearance surface. When temperatures are lower than standard, the aircraft’s true altitude will be lower than its barometric indicated altitudes.
1.2.2.2 Most existing VNAV systems do not correct for non-standard temperatures. At temperatures below standard, these errors can be significant and increase in magnitude as altitude above the station increases. The gradient of the approach obstacle clearance surface is reduced as a function of the minimum temperature promulgated for the procedure.
Note.— International Standard Atmosphere (ISA) temperature is 15°C at sea level with a lapse rate of 2°C per 1 000 ft of altitude.
1.2.3 Along-track position uncertainty
All RNAV systems have some amount of along-track error. This along-track uncertainty can mean that the VNAV system will start the descent too early and result in an error in the vertical path. This is compensated for in procedure design by relocating the threshold level origin of the approach obstacle clearance surface.
1.2.4 Flight technical error (FTE)
Flight technical error (FTE) is assumed to be contained within the standard non-precision margin of 75 m (246 ft). This is added below the VPA before the obstacle clearance surface is adjusted for cold temperature and along-track error.
1.2.5 Other system errors
Other errors include static source error, non-homogenous weather phenomena and latency effects. These are insignificant compared with the other errors already addressed and are considered as contained within the existing margin.
1.2.6 Blunder errors
Application of an incorrect or out-of-date altimeter setting, either by air traffic control or the pilot, is possible and must be prevented by appropriate operational techniques.
1.3 EQUIPMENT REQUIREMENTS
1.3.1 APV/baro-VNAV procedures are intended for use by aircraft equipped with flight management systems (FMS) or other RNAV systems capable of computing barometric VNAV paths and displaying the relevant deviations on the instrument display.
1.3.2 Aircraft equipped with APV/baro-VNAV systems that have been approved by the State of the Operator for the appropriate level of lateral navigation (LNAV)/VNAV operations may use these systems to carry out APV/baro- VNAV approaches provided that:
a) the navigation system has a certificated performance equal to or less than 0.6 km (0.3 NM), with 95 per cent probability. This includes:
1) GNSS navigation systems certified for approach operations;
2) multi-sensor systems using inertial reference units in combination with certified DME/DME or GNSS; and 3) RNP systems approved for RNP 0.3 operations or less;
b) the APV/baro-VNA V equipment is serviceable;
c) the aircraft and aircraft systems are appropriately certified for the intended APV/baro-VNAV approach operations;
d) the aircraft is equipped with an integrated LNAV/VNAV system with an accurate source of barometric altitude; and
e) the VNAV altitudes and all relevant procedural and navigational information are retrieved from a navigation database whose integrity is supported by appropriate quality assurance measures.
Note.— Acceptable means of compliance can be found in documents such as the United States Federal Aviation Administration (FAA) Advisory Circular (AC) 20-138, AC 20-130A and AC 120-29.
1.3.3 Where LNAV/baro-VNAV procedures are promulgated, the approach area has been assessed for obstacles penetrating the Annex 14 inner approach, inner transitional and balked landing surfaces. If obstacles penetrate these surfaces, a restriction is placed on the minimum value of OCA/H permitted (see 1.1.5).
1.4 OPERATIONAL CONSTRAINTS
1.4.1 Pilots are responsible for any necessary cold temperature corrections to all published minimum altitudes/heights. This includes:
a) the altitudes/heights for the initial and intermediate segment(s);
b) the DA/H; and
c) subsequent missed approach altitudes/heights.
Note.— The final approach path VPA is safeguarded against the effects of low temperature by the design of the procedure.
1.4.2 Temperatures below the promulgated minimum
Baro-VNAV procedures are not permitted when the aerodrome temperature is below the promulgated minimum aerodrome temperature for the procedure, unless the flight management system (FMS) is equipped with approved cold temperature compensation for the final approach. In this case, the minimum temperature can be disregarded provided it is within the minimum certificated temperature limits for the equipment. Below this temperature, and for aircraft that do not have FMSs equipped with approved cold temperature compensation for the final approach, an LNAV procedure may still be used provided that:
a) a conventional RNAV non-precision procedure and APV/LNAV OCA/H are promulgated for the approach; and
b) the appropriate cold temperature altimeter correction is applied to all minimum promulgated altitudes/heights by the pilot.
1.4.3 A VPA deviation chart may be published on baro-VNAV instrument procedure charts, correlating an aerodrome temperature with an associated true VPA. This chart is intended to advise flight crews that although the non- temperature compensated aircraft’s avionics system may be indicating the promulgated final approach VPA, the actual VPA is different from the information presented to them by the system.
1.4.4 This chart is not intended to have the pilot increase or decrease the VPA flown to achieve the actual promulgated VPA. A sample of the chart is provided in Table II-4-1-1.
1.4.5 Some baro-VNAV systems have the capability to correctly compensate the VPA of an instrument approach procedure following an input of the aerodrome (altimeter source) temperature by the pilot. Pilots operating aircraft with this feature active are expected to ignore the VPA chart and fly the system-corrected VPA guidance
I hope that helps. Doc 8168 contains a wealth of information including correct entry procedures for conducting GNSS approaches, something that many still misunderstand.
So, when sitting there in the middle of the night as you're hauling *ss across the NOPAC/SOPAC/NATS or POLAR, haul out the manual and have a look. That's flying!
Seasons Greetings and a safe flying for New Year 2016 to all.