CPL - Commercial Pilot Licence
Radio Navigation
ILS, RNAV and RNP: flying precise instrument procedures and approaches.
The PPL lesson explained how VOR, DME, NDB, ILS and GNSS work. At CPL level you use them to fly instrument procedures: DME arcs, holds, precision and non-precision approaches and performance-based navigation. This lesson covers the procedures and the limits that come with them.
1. Flying a DME arc
A DME arc keeps the aircraft at a constant distance from a DME station, for example "the 12 DME arc".
- Lead the turn onto the arc by about 0.5–1 NM, then fly a heading about 90° to the radial.
- Every 10° of radial, turn about 10° towards the station to stay on the arc (with a wind correction).
- If the DME increases, turn towards the station; if it decreases, turn away.
2. Holding
A hold is a racetrack pattern over a fix. A standard hold has right turns and 1-minute inbound legs at or below FL140 (1.5 minutes above). Maximum holding speeds (ICAO) are about 230 kt up to FL140, 240 kt to FL200 and 265 kt to FL340 (check the charts – some states differ).
Entries
The entry depends on your heading arriving at the fix, compared with the inbound course. Draw a line through the fix at 70° to the inbound course on the holding side:
| Sector | Size | Entry |
|---|---|---|
| Sector 1 | 110° | Parallel: fly outbound on the non-holding side for one minute, turn back to intercept the inbound course (or return to the fix) |
| Sector 2 | 70° | Offset (teardrop): fly outbound at 30° to the inbound reciprocal on the holding side, then turn onto the inbound course |
| Sector 3 | 180° | Direct: cross the fix and turn straight into the hold |
Adjust the outbound leg time to make the inbound leg the right length, and correct for wind on the outbound leg with about three times the inbound drift.
3. Approach types
| Type | Lateral guidance | Vertical guidance | Minimum |
|---|---|---|---|
| Precision (ILS, GLS) | Localiser | Glideslope | Decision altitude/height (DA/DH) |
| APV (RNP APCH to LNAV/VNAV or LPV minima) | GNSS | Baro-VNAV or SBAS | DA/DH |
| Non-precision (VOR, NDB, LOC-only, RNP APCH to LNAV) | VOR, NDB, LOC or GNSS | None (fly a constant descent angle) | Minimum descent altitude (MDA) |
Non-precision approaches are now flown as CDFA (continuous descent final approach) – a steady descent on the published angle instead of "dive and drive" – with a go-around at the MDA if the runway is not in sight.
4. ILS categories
| Category | Decision height | Minimum RVR |
|---|---|---|
| CAT I | 200 ft or more | 550 m (or visibility 800 m) |
| CAT II | 100–200 ft | 300 m |
| CAT IIIA | Below 100 ft or no DH | 175 m |
| CAT IIIB | Below 50 ft or no DH | 50–175 m |
CAT II and III need certified aircraft, trained crews, protected ILS areas and the airport's low-visibility procedures in force – usually with an autoland.
ILS coverage: the localiser is reliable to 25 NM within ±10° of the centreline (17 NM to ±35°); the glideslope to 10 NM. Beware false glideslopes above the real one (at about 6° and 9°): always intercept the glideslope from below.
5. Performance-based navigation (PBN)
PBN defines navigation by the accuracy an aircraft must achieve, rather than the sensor it uses. The number is the accuracy in NM that must be kept 95% of the flight time:
| Specification | Accuracy | Typical use |
|---|---|---|
| RNAV 10 (RNP 10) | 10 NM | Oceanic and remote routes |
| RNAV 5 | 5 NM | Continental en route (formerly B-RNAV) |
| RNAV 1 | 1 NM | SIDs and STARs (formerly P-RNAV) |
| RNP 1 | 1 NM with monitoring | SIDs and STARs |
| RNP APCH | 1 NM initial, 0.3 NM final | RNAV(GNSS) approaches |
| RNP AR APCH | Down to 0.1 NM | Special approaches in difficult terrain – needs authorisation |
The difference between RNAV and RNP: an RNP system monitors its own accuracy and alerts the crew when it can no longer meet it.
6. Augmentation and integrity
- RAIM: the receiver checks itself using an extra satellite; it must be predicted available for RNP approaches.
- SBAS (EGNOS in Europe, WAAS in North America): satellites broadcast corrections, allowing LPV approaches with ILS-like minima.
- GBAS: a ground station at the airport corrects GNSS for GLS precision approaches.
7. Airborne weather radar
The radar sends pulses in a narrow beam and shows returns from precipitation (not cloud itself, and not dry hail or CAT). Use tilt to scan different levels: tilt down to see the tops of the storms ahead, tilt up near the ground to avoid ground returns. Water droplets reflect strongly, dry snow weakly – a storm can be stronger than it looks at high level.
Flying it at Levant
Practise holds and entries on IVAO – controllers often ask for them. Fly the published missed approach, intercept the ILS from below, and look at the minima box on the chart before every approach: the DA for an ILS, the MDA for a VOR or RNAV LNAV approach. Many Middle East airports publish RNP approaches next to the ILS.
Check yourself
- What are the standard holding direction and inbound leg time below FL140? Right turns, 1 minute.
- Which entry covers the 70° sector? The offset (teardrop) entry.
- What is the decision height for a CAT I ILS? 200 ft or more.
- Why intercept a glideslope from below? To avoid capturing a false glideslope above the real one.
- What does an RNP system do that RNAV does not? It monitors its own accuracy and alerts the crew.
Simulation training material. Always use current approach charts and your aircraft's procedures.