This page discusses the possible meaning of Unable RNP and possible implications for Air Traffic Controllers
There are several reasons why pilot(s) may declare 'Unable RNP'. The controller must be aware of why this could happen and what are the possible contingency solutions. A table of possible causes and appropriate mitigations is given below.
It is important to understand that 'Unable RNP' does not necessarily mean that the aircraft can not navigate or that certain functionalities (such as RF) have been lost. The controller will need to assess the situation and take the appropriate actions.
|
Problem |
Potential contingency solution | ||
|
Possible Cause |
Sample RTF report |
IMPACT |
ACTION |
|
Airborne receiver failure |
Single A/C CLEARED FOR APP says ‘Unable RNP |
RNP APCH procedure not usable by single aircraft |
Clear this aircraft for alternative procedure e.g. ILS |
|
Total GNSS signal loss (over an area) – duration unknown. |
Several aircraft aligned for APP say ‘Unable RNP’ |
RNP APCH procedure not usable by all aircraft |
Clear all aircraft for alternative procedure e.g. ILS |
|
Aircraft has FMS failure |
One or more A/C ON PBN STAR says ‘UNABLE RNAV’ (Radar Environment) |
Flight(s) continue(s) if conventional NAV available throughout |
Controller to issue Radar Vectors (unless flight crew states that conventional procedures possible). Potentially, traffic flow management regulation |
|
Total GNSS signal loss (over an area) |
Flight(s) turn(s) back if above n/a |
||
|
Aircraft with single FMS has FMS failure |
A/C ON PBN STAR says ‘UNABLE RNP’ (Radar Environment) |
Aircraft cannot fly STAR |
Controller Radar Vectors |
|
Total GNSS signal loss (over an area) |
Some aircraft cannot fly STAR others can continue normally. |
Controller Radar Vectors who cannot fly STAR; no action for those aircraft able to fly RNAV STAR |
|
|
Aircraft has FMS failure |
One or more A/C ON PBN STAR says ‘UNABLE RNAV’ (Radar Environment) |
Flight(s) continue(s) if conventional NAV available throughout |
Controller to issue Radar Vectors (unless flight crew states that conventional procedures possible). Potentially, traffic flow management regulation |
|
Total GNSS signal loss (over an area) |
Flight(s) turn(s) back if above n/a |
||
|
Aircraft with single FMS has FMS failure |
A/C ON PBN STAR says ‘UNABLE RNP’ (ADS-B only Environment) |
Aircraft cannot fly STAR |
Vectoring provided using ADS-B |
|
Total GNSS signal loss (over an area) |
Some aircraft cannot fly STAR others can continue normally. |
Procedural Control (local implementation) and potentially, traffic flow management regulation |
|
|
Aircraft has FMS failure |
One or more A/C ON PBN STAR says ‘UNABLE RNAV’ (ADS-B only Environment) |
Flight(s) continue(s) if conventional NAV available throughout |
Vectoring provided using ADS-B
|
|
Total GNSS signal loss (over an area) |
Flight(s) turn(s) back if above n/a |
Procedural Control (local implementation) and potentially, traffic flow management regulation |
|
|
Aircraft has FMS failure |
On or more aircraft on Free Route says ‘Unable RNAV’ or ‘Unable RNP’ |
Single aircraft cannot continue cleared flight path. |
Controller to issue Radar Vectors and continue to maintain separation assurance. |
|
Total GNSS signal loss (over an area) |
No aircraft can continue on cleared trajectory |
Conventional navigation if available or Radar Vectoring and Traffic Flow Management regulation |
|
|
Note: there may be several causes for UNABLE RNP above those included in this table. E.g. jamming or spoofing of the GNSS signal(s); signal interference (e.g. locally, unintentional), space weather. |
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Problem |
Potential contingency solution |
|
|
IMPACT |
ACTION |
|
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Credible RNP corruption (RNAV/RNP navigation avionics failure)
|
Single aircraft inadvertently deviates from planned trajectory (this case is not considered credible for multiple aircraft)
|
Deviation is detected by ATC and reported to the aircraft, after which an UNABLE RNP situation ensues.
|
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Credible RNAV/RNP corruption (data integrity failures in either procedure designs, AIS data or nav databases)
|
Single or multiple aircraft inadvertently deviate from planned trajectory. Deviations not likely to be simultaneous.
|
Deviation is detected by ATC and reported to the aircraft, after which an UNABLE RNP situation ensues. Possible NOTAM and/or ATIS warnings issued for awareness of other aircraft.
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Credible RNAV/RNP corruption (GNSS integrity failure/GNSS signals spoofed)
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Multiple aircraft inadvertently deviate from planned trajectory. Several aircraft deviate simultaneously.
|
Deviation is detected by ATC and reported to the aircraft, after which an UNABLE RNP situation ensues. Possible NOTAM and/or ATIS messages issued for awareness of other aircraft.
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