The vertical challenge

By Anthony MacKay, Director, Operational Safety, NAV CANADA

When I started flying in 1986, the definition pertaining to precision and non-precision approaches was very simple. Precision was ILS and everything else was non-precision where dive and drive was the way you managed the vertical. Today, with the advent of RNAV, the precision/non-precision line is very blurry. Let us consider the following approaches:

Conventional Approaches

• ILS
• LOC
• NDB
• VOR

RNAV Approaches

• LPV
• RNP
• L/V
• LNAV/VNAV
• LP
• LNAV

Now let’s list them by their ability to provide electronic vertical guidance and pilot derived vertical guidance via vertical speed or flight path angle:

Approaches with vertical guidance

• ILS
• LPV
• RNP
• L/V
• LNAV/VNAV

Approaches without vertical guidance

• LOC
• NDB
• VOR
• LP
• LNAV*

*may have advisory BARO VNAV

The approaches with vertical guidance can be further broken down to those that provide geometric vertical guidance and those that provide barometric vertical guidance:

Geometric vertical guidance

• ILS
• LPV
• L/V

Barometric vertical guidance

• RNP
• LNAV/VNAV
• LNAV*

*if advisory BARO VNAV is used

Geometric vertical guidance maintains a constant angle relative to the runway regardless of altimeter errors due to non-ISA temperatures, mountain wave effect or altimeter setting errors. As an example, during the winter at -35°C when flying the ILS or LPV to runway 12 in Edmonton, the aircraft, when on the glide path, will cross the FAF at the correct true altitude, but the indicated altitude will be higher than normal. During the summer at 35°C when flying the ILS or LPV to runway 12 in Edmonton, the aircraft, when on the glide path, will cross the FAF at the correct true altitude, but the indicated altitude will be lower than normal. If temperature corrections have been completed, this will be expected.

ILS 12

RNAV 12 (LPV)

ISA FAF Altitude and Approach Angle	Indicated Altitude -35°C & Approach Angle	Indicated Altitude 35°C & Approach Angle
ILS 3650’ & 3°	3893’ & 3°	3548’ & 3°
LPV 3600’ & 3°	3833’ & 3°	3502’ & 3°
L/VFootnote 1 3600’ & 3°	3833’ & 3°	3502’ & 3°

Barometric vertical guidance will shallow (cold) or steepen (warm) the approach angle relative to the runway due to altimeter errors like non-ISA temperatures, mountain wave effect or altimeter setting errors. As an example, during the winter at -35°C when flying the LNAV/VNAV or LNAV (*with advisory BARO VNAV) to runway 12 in Edmonton, the aircraft, when on the barometric vertical path, will cross the FAF at the correct indicated altitude, but the true altitude will be lower than designed. During the summer at 35°C when flying the LNAV/VNAV or LNAV (*with Advisory BARO VNAV) to runway 12 in Edmonton, the aircraft, when on the barometric vertical path, will cross the FAF at the correct indicated altitude, but the true altitude will be higher than designed. RNP procedures use barometric vertical paths and are subject to the same errors. The examples below show the effect without temperature compensation applied.

RNAV 12 (LNAV/VNAV or LNAV (*with advisory BARO VNAV)

ISA FAF indicated altitude and approach angle	True altitude -35°C & approach angle	True altitude 35°C & approach angle
LNAV/VNAV 3600’ & 3°	3367’ & 2.5°	3698’ & 3.2°
LNAVFootnote * 3600’ & 3°	3367’ & 2.5°	3698’ & 3.2°

The good news is that if your flight management system (FMS) has a temperature compensation function, and you use it, the FMS will correct the aircraft vertical path for non-standard temperatures back to the correct true altitude and approach angle. Currently, aircraft procedures are developed for temperature compensation and cold temperatures, but very little guidance exists regarding higher than ISA temperatures on approach. For aircraft operators using FMS that do not have temperature compensation, cold temperature limits are included on the approach plate limiting the use of the approach to temperatures warmer than the published limit.

Additional good news is that Transport Canada is requiring that all new aircraft with FMS BARO VNAV have temperature compensation. “New or updated FMS designs shall provide a means for an aircraft to fly the true vertical path angle for final approach segment, as defined in the resident navigation database, in below ISA temperature conditions.” Transport Canada AC 500-020 provides additional information.

The CAP plate shows no vertical information.

The Jeppesen Plate shows 3.49° which matches the FMS coding.

But to clear obstacles in the visual segment of the approach to RWY 20 in Dease Lake, you have to fly the 4.0° PAPI in the visual segment of the approach.

If an LNAV procedure is collocated with an ILS, LPV, RNP or LNAV/VNAV procedure, the visual segment of the approach will be protected on the BARO VNAV path. If the LNAV procedure is stand-alone such as the Dease Lake example, the PAPI will provide the visual segment obstacle protection.

Finally, if using an SCDA technique (VS or FPA) on LP, LNAV, LOC, NDB or VOR procedures, the requirements of Transport Canada AC 700-028 must be adhered to.