This section provides your questions and our responses:

11 March 2019

What is the difference between RAIM or AAIM and the On Board Alerting and Monitoring System of RNP?

The simple answer is that Receiver Autonomous Integrity Monitoring (RAIM) or Aircraft Autonomous Integrity Monitoring (AAIM) are ways of providing confidence that the GPS receiver is providing a good position estimation; the monitor will provide an alert if it detects an erroneous satellite signal that could degrade the GNSS position estimation.  In RAIM, simply put - this is achieved by comparing position estimations between different sets of 4 satellites provided additional satellites (ie 5+) are in view; five satellites will enable Fault Detection (FD) and 6+ will enable Fault Detection and Exclusion (FDE).

On-board Performance Monitoring and Alerting (OPMA) is a functionality required of all RNP aircraft.   This functionality will ensure that an alert is provided to the pilot if, in normal operations, the aircraft exceeds the Required Navigational Performance for that ATS route.  This means that the Actual Navigation Performance (ANP) exceeds the RNP.  In normal operations (ie everything is working correctly) the probability of that alert not being given is 10-7 per flight hour (one time in 10 000 000).  For abnormal circumstances (ie when there are problems with the avionics), we bound the uncertainty in position estimation by stating that if it exceeds 2 x RNP, the system should give an alert; the failure of this alert not be given is categorised as major – one missed detection in 100 000 per flight hour (10-5 ).

Therefore, RAIM or AAIM provide integrity on the position estimation.  This provides the confidence that the navigation system is operating correctly.  OPMA provides the pilot with an alert if the aircraft is not meeting the stipulated performance requirement of the ATS route when the avionics is operating correctly.  However, if the aircraft system or navigation solution is degraded, then OPMA should alert when uncertainty in the position estimation exceeds 2 x RNP; the probability of the pilot not getting this alert is set as a major failure condition (ie 10-5 per flight hour).


6 Mar 2019

As of 30 January 2020, Tango 9 will become RNP 2.  All our aircraft are capable of RNP 2 operations, all crew will be qualified under our current PBN qualification (with minor SOP change required re FMC RNP settings).

To operate RNP 2 ‘en-route’ we will require an en-route RAIM check what are the requirements?

From the PBN Manual, Volume II, Part C, 'Implementing RNP 2' chapter: The RNP 2 specification is based upon GNSS. Operators relying on GNSS are required to have the means to predict the availability of GNSS fault detection (e.g. ABAS RAIM) to support operations along the RNP 2 ATS route. The on-board RNP system, GNSS avionics, the ANSP or other entities may provide a prediction capability. The AIP should clearly indicate when prediction capability is required and an acceptable means to satisfy that requirement.

UK NATS have informed us of the following:

  • 30th January 2020 sees the expansion of the NAT DLM and ceasing the Tango exemptions.
  • 30th January 2020 we will re-designate Tango 9 as an RNP2 (offshore) route at FL290+.
  • 30th January 2020 we will introduce a Tango 290 (20NM west of Tango 9) as an RNP2 (offshore) route at FL290+.
  • From 30th January only flights that are RNP2, VHF and ADS-B out will be permitted on those route at FL290 and above.

Tango 9 and Tango 290 considered RNP 2 Continental (offshore) routes so low continuity required and ac must have at least a single GNSS LRNS.  Loss of GNSS is classified as a minor failure condition provided the operator can revert to a different navigation system and proceed to a suitable airport.  INS as an alternative system acceptable and flight plan data shows NATS that there is a 99.8% equipage with INS.

Flight planning requirements for filing T 9 and T 290 will be:

  • Item 10A – G/R/Y/Z
  • Item 10B – B1 or B2
  • Item 18 – NAV/RNP2.

There is no change to the existing RAIM requirements placed on airlines.

You can expect to see updates to the UK AIP with regards the new routes in December this year, a minimum of 2 AIRAC cycles ahead of the change, until then we have an AIC out which introduces the VHF channel on the existing Tango 9.


01 Mar 2019

A business jet operator with aircraft equipped with SBAS and approved to fly LPV procedures but the operator does not have approval for LNAV-VNAV approaches. Can they use SBAS for LNAV-VNAV approaches when the airport does not promulgate LPV minima on the charts?

It depends on the type of equipment, the manufacturer and to a certain extent to the airworthiness certification.

SBAS capable receivers come in four functional classes (1-4) and three operational equipment classes (Beta, Delta, Gamma). Functional classes 1 and 2 are virtually non-existent. Class Beta 3 and Class Gamma 3 support approaches to LNAV, LNAV/VNAV, LP and LPV minima. Whereas, Class Delta 4 only supports approaches to LP and LPV minima.

So if the airplane is equipped with a Class Beta 3 or a Class Gamma 3, it should be technically capable of operating to LNAV and LNAV VNAV minima.

Have all aircraft carrying a Beta 3 or Gamma 3 receiver been certified to operate to LNAV or LNAV/VNAV minima? Apparently not.  Some applicants did not apply for this, even though EASA has encouraged them to do so.  Therefore, on those aircraft, you will not find an AFM statement stating capability to perform an RNP APCH procedure to LNAV or LNAV/VNAV minima.

It is noted that there is one loophole in the EASA guidance to OPS regulations that could still allow these aircraft to fly to LNAV/VNAV minima, as it does allow aircraft certified to AMC 20-28 (indeed, the AMC for RNP APCH to LPV minima) to also operate on procedures to LNAV and LNAV/VNAV minima.  However, this is subject to acceptance by the competent authority having oversight of flight operations; in short, the NAA provided that they are equipped with a Beta 3 or Gamma 3 receiver/navigator. Please note that this does not apply to Delta 4 equipment which does not qualify.

18 Feb 2019

What can be the reason that – sometimes – the LPV VNAV minima are lower than the LPV SBAS minima? Normally the opposite is true.

As far as I understand, the VNAV minima are based on the baro alt and the SBAS minima are based on the GPS altitude.

For example, when looking at Emden (Germany), the VNAV minima are lower than the SBAS minima. In poor weather conditions with low ceilings, it would be advantageous to continue the approach when switching off SBAS. This can’t be the purpose of course.

Your help would be much appreciated.

A LPV minima should normally be better than a LNAV/VNAV minima. As you correctly said geometric altimetry (APV SBAS to a LPV minima) is normally going to be more accurate than barometric altimetry (APV Baro to a LNAV/VNAV minima).

Our initial thought was that there was a difference/anomaly in the two design criteria.  We have seen this before with LNAV versus LNAV/VNAV design criteria; here, in some cases, you can get a lower minimum with just a 2D path (LNAV) than you can with a 3D path (LNAV/VNAV) – this should not be the case as a guided vertical path must surely be safer.  The ICAO Instrument Flight Procedures Panel (IFPP) fully accepts this and are looking at the two design criteria.

However, on checking with DFS, we have been informed that it is not an anomaly in the design criteria.  Apparently, DFS has decided that unless the aerodrome has CAT I infrastructure, they will apply the LNAV minima to the LPV procedures.

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