Course Tolerances & Deviations

Tizi
Apr 5
5 min read

"You're off by 1.5 degrees" - SAV ATC "Thanks." - Tizi

From your first day under instruments, you're trained to be super precise. And that is appropriate - the ACS has clear tolerances on where you're supposed to be at all times. It's the nature of IFR - being exactly where you're supposed to be so ATC can coordinate positive separation from other aircraft.

Tolerances

When talking about tolerances, we need to clarify that there's a difference between airway "protected area" and how far from course you're allowed to deviate.

Airway Protected Area

Traditional VOR-based Victor Airways are designed to provide an 8 NM protected area, called primary obstacle clearance area. This is 4 NM to the left and 4 NM to the right of your course.

They offer 1,000 ft obstacle clearance in non-mountainous terrain and 2,000 ft in mountainous areas.

If you're nerdy, you can rejoice in knowing that the 4 NM is calculated from a +/- 4.5 degree radial accuracy outbound from the VOR (in statistical terms, +/- 4.5 deg accuracy 95% of the time). That 4.5 degree angle outbound reaches a lateral distance of 4 NM when at 51 NM away from the station (see the image above from the FAA's Instrument Procedures Handbook). In essence, if the inaccuracy of the VOR can be up to 4 NM left or right (or 8 NM total), then the FAA must protect the airspace all the way to that inaccuracy point.

The MEA and MOCA are applicable to 4 NM left and right of course.

The more perceptive of you will however note that an airway can have a leg to a changeover point longer than 51 NM. This is where the 4.5 degree error valid for 95% of the time is insufficient. We need a stronger statistical value. Statistically, the VOR will have an accuracy of +/- 6.7 degrees 99% of the time. So, a secondary obstacle clearance area is formed, extending extra 2 NM in each direction. That means a total of 6 NM in each direction, or 12 NM total width, as shown in the image below. However, the secondary protection areas don't offer a clear obstacle clearance like the primary areas, so I recommend you just consider the 4 NM in each direction as your "protected area".

When the total airway distance is greater than 102 NM, the angles of 4.5 deg and 6.7 deg that form the geometry for the primary and secondary protection areas extends further than the 4 NM (and extra 2 NM). In that case, it may form a flare (extra area of protection), that is illustrated in the image below.

This is great information, but a little beyond operational value. As such, just remember that you are protected to 4 NM in each direction along an airway. But you should always have your needles centered - so this should not be a consideration as you fly.

For Tango routes, the FAA maintains the same standards for VOR-based routes, even though the problem of reception no longer exists. So, consider 4NM in each direction and you're fine.

Airway Protected Area: Direct Routes (Off-Airways)

When off airways, pilots are responsible for maintaining obstruction clearance. Refer to the Off Route Obstruction Clearance Altitude (OROCA) for appropriate altitudes to fly. The OROCA offers you 1000 ft clearance in non-mountainous terrain and 2000 ft in mountainous areas.

Acceptable Course Deviations

The FAA is (surprisingly) not very prescriptive on how much you can deviate off course. 14 CFR 91.181, reported below, indicates that you must always be on centerline, whether on an airway [91.181(a)] or on any other route [91.181(b)].

In AIM 5-5-16, a definition of "established" is reported for RNAV/RNP operations:

That is a hint to how much we can deviate, but it is not a requirement for precision. Both T routes (IFR low structure) and Q routes (IFR high structure) have a NAV specification of RNAV 2. Therefore, you are established when you're within 2 NM of the centerline. But established doesn't provide us with a standard or acceptable limit.

When trying to answer this question, I consider three pieces of information:

The Instrument Rating Airmen Certification Standards (ACS)
AIM 5-5-5
Flight Safety Foundation's ALAR (Stabilized Approach Information)

The ACS requires students to be within 3/4 scale deflection at all times. Any deviation more than 3/4 scale is considered a failure. As we will learn in the next section, this deviation changes (on GPS) depending on the phase of flight, but even for the largest scale (which is 5 NM), you would be deviation 3.75 NM off course. So you're at the edge of the 4 NM airway protection area. That feels like a justified answer.

AIM 5-5-5 talks about approaches, but it says that you must execute a missed approach if you are not in a safe position to land the airplane. If you are full deflection, you techically don't know "exactly" where you are. So a safe landing cannot be guaranteed.

The ALAR document (see reference here on stabilized approaches), talks about 1/2 a deflection on an ILS. If we were to examine the requirements for ILS standards, we would find in 6750.16E that a full scale CDI must match a specific frequency which matches the edge of the localizer beam. So, if you go full deflection, you may really be "off the localizer" which means off course.

Since everything in aviation is about safety, my personal opinion is that you should follow the ACS, and consider yourself off course if you are more than 3/4 scale deviation off centerline. That is because it gives you - in the enroute world - just a minor buffer before you exit the protected area. If you are full deflection, you should definitely go missed because you technically "don't know where you are".

That doesn't mean that ATC won't scream at you before that...

CDI Deviations

Your Course-Deviation Indicator (CDI), whether on an HSI or a standalone instrument, provides you with guidance to stay on course. The deflection of the needle left and right, measured in "dots", can change based on the phase of navigation. Other ground-based navigation have other requirements. Please refer to your avionics manual for exact details.

GPS CDI Scaling

When using GPS to navigate, your CDI deflection scale changes based on phase of flight. The table below reports the scale changes for Garmin avionics.

Phase	Definition	Full Deflection Scale	Illustration
Enroute ENR	>30 NM from destination	5.0 NM or 2.0 NM
Terminal TERM	< 30 NM from destination	1.0 NM
Approach LNAV	< 2NM from the Final Approach Fix (FAF)	0.3 NM
Approach LNAV/VNAV	< 2 NM from the Final Approach Fix (FAF)	0.3 NM
Approach LNAV/VNAV	< 2 NM from the Final Approach Fix (FAF)	?	?
Approach LP	< 2 NM from the Final Approach Fix (FAF)	~2.5 degrees or as needed to provide 350 ft full deflection at runway threshold Source: RTCA DO229D
Approach LPV	< 2 NM from the Final Approach Fix (FAF)	~2.5 degrees or as needed to provide 350 ft full deflection at runway threshold Source: RTCA DO229D
Approach LPV	< 2 NM from the Final Approach Fix (FAF)	0.7 degrees
Missed Approach MAPR	At Missed Approach Activation	0.3 NM