Oceanic Errors Crossing NAT HLA



How Many Aircraft are Crossing NAT HLA?

This perfectly reasonable and simple question does not have a simple answer. Traffic data is collected by each individual oceanic control area but because of the way in which the data is reported it is impossible to answer the question. In short, this is because one traffic movement may involve an aircraft transiting as little as one or as many as 3-4 oceanic control areas. It may (or may not) fly for some of its journey within the OTS and the remainder within random routing.

Then there is the question of what constitutes a flight?  Do we include traffic at any level or just within the RVSM/NATHLA band? Where does that leave other traffic that might climb through those levels to above F410? Do we include (or omit) the light single on a transatlantic ferry flight, or sub-hunting P3 operating at low level?

Even estimating an annual number is difficult to find traffic numbers (or even estimates) on the Internet.  The only data reported to the NAT Central Monitoring Agency, NAT CMA is that provided by Shanwick OCA for traffic "eastbound crossing 20˚W between 45˚N and 61˚N, and this captures approximately 80-85% of the total estimated traffic. With this reporting criteria… Approximately 460,000 flights cross the North Atlantic each year. 

With this reporting criterion as a baseline, we have an opportunity to view comparative variations in traffic and clearance deviations on a seasonal basis. Just like everybody thinks THEY are an above average car driver when driving around the city, most of us just KNOW that a oceanic error will happen to the other pilot. Let the facts show you where the threat is.

Who is doing all the Monitoring in NAT HLA?

NAT Central Monitoring Agency is located adjacent to the Shanwick Oceanic Area Control Centre, in Prestwick, Scotland. Operational performance monitoring is one of the principle tasks of the NAT CMA. The primary source of operational safety reports are the air traffic control units responsible for the Oceanic Control Areas in the region, augmented by operator reports as available, (think airline captain’s reports). The CMA reviews the reports and a de-identified summary is available at year’s end. 

What Defines a Clearance Deviation in NAT HLA?

First off, let’s define what is reportable to you POI from the NAT CMA. In 2009 the FAA changed the terminology. The terms Oceanic Navigation Error Report (ONER), Oceanic Altitude Deviation Report (OADR), and Erosion of Longitudinal Separation (EOLS) have been changed to harmonize with ICAO reporting terminology.

  • Gross Navigation Error, GNE replaces ONER
  • Height Error replaces OADR
  • Time Error replaces EOLS
  • FAA Added the term “Intervention” meaning ATC prevents the deviation

Here is the reportable list of events listed in NAT Document #007, Chapter 11.

  • Gross Navigation Errors (>25NM from cleared route)
  • Lateral Deviations of Less than 25NM
  • ATC Interventions to Prevent Lateral Deviations
  • Large Height Deviations (+/-300ft from assigned Flight Level)
  • Erosion of Lateral or Longitudinal Separation
  • Time Errors (3 mins or more between ETA and actual time at a boundary or waypoint)
  • Speed Errors – any change of Mach No. or speed that results in a Time Error as above
  • Air Traffic Control inter-agency coordination errors
  • Communications failures
  • Non–approved (MNPS and/or RVSM) or inappropriately equipped aircraft flying at RVSM levels
  • Diversions & Turnbacks
  • Adoption of a published contingency procedure


There are several points here that will need some clarification

  • ATC Assigned Mach number adherence
  • GNE 25NM threshold
  • ETA’s and the “3Mins or more” quote.


ATC Assigned Mach Number vs. Flight Plan Mach Number

Once operating in Oceanic/Remote airspace it is helpful to break this question down into category of aircraft without an assigned Mach number (Flight Plan Mach) and aircraft with an ATC assigned Mach number.

Without an assigned Mach number from ATC, pilots are expected to fly the flight plan Mach number or whatever speed the pilot updated with ATC with. Some speed variance is allowed to this number. Here is the worldwide standard from ICAO Annex 2, Paragraph, published in 2012:

“Variation in true airspeed: if the average true airspeed at cruising level between reporting points varies or is expected to vary by plus or minus 5 per cent of the true airspeed, from that given in the flight plan, the appropriate air traffic services unit shall be so informed.”

Here is what the USA publishes in the AIP Enroute Section, Paragraph 42.7.5

Pilot (In Oceanic Class A and E Airspace)

“ If ATC has not assigned an airspeed, advises ATC anytime the true airspeed at cruising level varies or is expected to vary by ±10 knots or 0.02 Mach number, whichever is less, of the filed true airspeed.”

With a little bit of math, it’s easy to see that the US criteria is more restrictive and will become the defining criteria here.

ICAO= +/- 5% of 460KTAS = 23KTS

USA= +/- .02 of 460KTAS=  9.2KTS (<10KTS)

Once ATC has assigned a Mach number to your aircraft, there is no “Plus-or-Minus” to the speed. ATC is using a “Mach Number Technique” to keep your aircraft separated and is relying on you to maintain that speed for the required separation.

This is what ICAO has to say on the subject, Document #4444, Paragraph “Longitudinal Separation Minima With Mach Number Technique Based On Time. Turbojet aircraft shall adhere to the true Mach number approved by ATC and shall request ATC approval before making any changes thereto. If it is essential to make an immediate temporary change in the Mach number (e.g. due to turbulence), ATC shall be notified as soon as possible that such a change has been made.”

Here is what the FAA is expecting you to do once ATC has assigned a Mach number to your aircraft. This quote comes from the USA’s AIP, Enroute Section, Paragraph

“If ATC has assigned an airspeed, aircraft must adhere to the ATC assigned airspeed and must request ATC approval before making any change thereto. If it is essential to make an immediate temporary change in the Mach number (e.g., due to turbulence), ATC must be notified as soon as possible. If it is not feasible, due to aircraft performance, to maintain the last assigned Mach number during an en route climb or descent, advises ATC at the time of the request.”

Seems pretty straightforward, if ATC assigns a Mach number to maintain, maintain that Mach number. If no speed is assigned by ATC, fly the flight plan Mach +/- less than 10KTAS.

There is lingering confusion on this subject. Oakland Oceanic, KZAK has a NOATM to the effect of reporting any changes in Mach number that are in excess of .02 the Mach number at FIR entry; or any subsequent speed change notified to ATC in flight. (NOTAM#A1613/16). This is to accommodate LRC or ECON speed variances. It is critical to note that ATC still needs notification of the changes and this procedure only applies only to operations in KZAK and not for flights into the NAT.

One more point about speed. The Mach number from ATC is a True Mach number not an Indicated Mach number. Some airframes will need to do a conversion to achieve the correct ATC assigned True Mach number.

25NM vs. 10 NM

A GNE within NAT Airspace is defined as a deviation from cleared track of 10 NM or more (note the change from ≥ 25 NM post implementation of the RLatSM trials). A GNE is defined by the FAA as a deviation from cleared track of 25 NM or more WORLDWIDE. These errors are usually detected by means of long-range costal radar as aircraft leave oceanic airspace. Other such errors may also be identified through the review of routine position reports such as ADS-C from aircraft.

The 10NM definition is not accepted as a worldwide standard at ICAO. The FAA has this under study BUT has not accepted the 10NM definition. Take a look at FAA Order 8900.1, Volume 7, Chapter 3, Paragraph 7-81 for details on this.

2 Minutes or 3 Minutes

Here is the worldwide standard from ICAO Annex 2, Paragraph;

“Change in time estimate: if the time estimate for the next applicable reporting point, flight information region boundary or destination aerodrome, whichever comes first, is found to be in error in excess of 2 minutes from that notified to air traffic services, or such other period of time as is prescribed by the appropriate ATS authority or on the basis of air navigation regional agreements, a revised estimated time shall be notified as soon as possible to the appropriate air traffic services unit.”

ICAO modified Annex 2 with Amendment #43 in 2012 with this newer 2-minute/120 second threshold so as to be consistent between voice and ADS-C reporting. That being said, some regulators describe this reporting requirement as "3 minutes or greater" vice the “Greater than 2 minutes” found in Annex 2. It is the same threshold… just described from the other end of the number line. From a Human Factors viewpoint, the choice to use "3 Minutes or greater" is poor. It too closely resembles the old definition and creates the confusion we see now.

It helps to keep the reason for this change in mind, ADS-C reporting. One of the ADS-C reporting parameters that ATC will set is "120sec". The contract parameters are mostly hidden from the operating pilot. Buried inside the FMS, there are displays that will allow a pilot to view exactly ATC has "Contracted" for with your FMC. An ETA revision of greater than 120 seconds is one of these triggers. With ADS_C reporting, voice reports are not required. Reporting is automatic thru ADS-C updating ATC anytime your “NEXT” estimated time of arrival is off by more than 120 seconds.

Where are the “Hot Spots” in NAT HLA?

More than one per day, every day some pilot will leave their ATC clearance. Some of these are intentional such as weather avoidance or medical diverts. Some are “blunder” errors. Deviations usually run about 30/month until summertime. June and July are the highest frequency of deviations, 50-60/month…over two-times the monthly average!

Large Height Deviations, non-compliance with a conditional clearance and followed flight plan instead of the clearance are the big three causes. Incorrect execution of an oceanic weather deviation is another large cause.

Shanwick OCA has the largest portion of NAT traffic AND the largest number of deviations. Especially in the southeastern corner. Deviations on random routes are more common than inside the OTS.

AFIS/ACARS clearance delivery is more prone to deviations than CPDLC or Voice clearance delivery


1. If you get an assigned a speed from ATC, hold that speed +/- 0

2. FAA still uses the 25NM definition for a GNE. NAT CMA is tracking/reporting all deviation in excess of 10NM for statistical purposes because of the RLatSM implemented in late 2015.

3. Worldwide if you observe an ETA in excess of 2minutes/120sec difference, update ATC with the revised estimate.

4. June and July are the busiest months. In the NAT HLA. Shanwick OCA is the busiest portion of the NAT HLA and has the most clearance deviations in the NAT HLA.

5. The Southeast corner of Shanwick OCA is especially problematic.

6. Poorly executed weather deviations are as bad as blundering off your ATC clearance

7. Random routes have more clearance deviations than OTS routing.

8. Large Height Deviations, LHD are more common than Gross Navigation Errors, GNE

9. Oceanic clearances received via AFIS/ACARS are more problematic that CPDLC or Voice.

10. Following the flight plan instead of the ATC clearance and/or not complying with a conditional clearance remain prime problems in the Shanwick OCA.