What limits define a "deviation"?

moxiepilot

moxiepilot

Well-Known Member
I was quickly thumbing through the FAR AIM and hadn't come across the specifics of a scenario I was thinking about (either that or my brain is fried from reading it cover to cover in 2 days)

So, here it is:

Besides the PTS for instrument flight where is it noted the allowed or "acceptable" deviation from a clearance?

For example if you are given a heading and altitude to be flown, where is it listed that a 150' gain in altitude or 15 degree heading change is an unauthorized deviation from your clearance?

What I'm asking is: where are the tolerances listed? My brain is fried so any help is appreciated.
 
i'm thinking that any published tolerances would be found in the atc operational procedures. part 91 for ifr only references 91.179, which states the 'exact altitudes' to fly, without mention of deviations. same with 91.181, which defines 'exact courses' to be flown, same with no mention of 'tolerances'. atc will always advise you when you do deviate..typically over the years i've heard it if off airway by like a half-mile or several hundred feet in altitude. and i think they 'may' use their discretion concerning some of this depending upon traffic/separation with other aircraft within your proximity. hope this helps? also, you may request block altitudes, particularly in turbulent conditions, or request heading deviations for weather or the rarely used clearance to 'remain in vmc conditions',which is basically a lateral ifr clearance, giving you discretion to deviate as necessary at will. it's particularly useful when working around weather with embedded thunderstorms.
 
yes it is helpful. for years I've been thinking along the lines of +/- 100 altitude, but now that I'm thinking about things in a different way, that's what you're tested on, not what the regs state.

I agree with you on 91.179 & 181, but they do not mention anything other than a hard clearance altitude or course.

Interesting that I had a discussion with an FAA DPE the other day about why the FAR AIM exists. Most people, like me, think it is created in the interest of safety. Silly me. I found out that it exists so that the FAA does not get sued. Hmm.

Put in this context, I can see how this is the case. Not flying at the altitude we told you? You got hurt? So sue us! We told you so... :D
 
moxiepilot said:
yes it is helpful. for years I've been thinking along the lines of +/- 100 altitude, but now that I'm thinking about things in a different way, that's what you're tested on, not what the regs state.

that's correct.




I agree with you on 91.179 & 181, but they do not mention anything other than a hard clearance altitude or course.
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yeah, it's what i was saying in a roundabout way.. :)
 
If the question is what will the FAA consider a deviation for enforcement purposes, it's not totally clear. I'm not aware of a reference that says that a deviation up to XXX feet is okay. No solid reference but I understand that the "trigger point" is 200' for altitude - mostly because that when the Operational Error Detection Program (OEDP) computer authomatically produces an alert on the controller's screen. That's going to make the controller pay attention.

Beyond that, from both an ATC reporting standpoint and an FAA enforcement standpoint, it will depend on the severity of the deviation and the impact on air traffic - in other words, was there at least a potential loss of separation? The FAA's enforcemet program had a guidelient for many years that You are correct that once reported, the FAA inspector handling the case consider only administrative action (like a warning letter) action for a deviation of 500 feet or less with no near-miss. But the enforcement guidelines were rewritten 2 months ago and this language no longer appears.
 
This probably doesn't help too much, but I remember my DPE telling me during my IFR oral that ATC will advise a pilot to check their altitude when they are ±300 feet of their assigned altitude. I did a quick search online and saw that referenced in this article at AOPA: (emphasis added by me)
The controller's responsibilities for this flight are dictated under the definition of "Additional Services" in the Air Traffic Control Handbook: "Advisory information provided by ATC which includes but is not limited to the following: traffic advisories; vectors, when requested by the pilot; altitude deviation information of 300 feet or more from an assigned altitude as observed on a verified automatic altitude readout (Mode C); advisories that traffic is no longer a factor; weather and chaff information; weather assistance; bird activity information; and holding pattern surveillance.
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I figure unless it's buried in the FARs/AIM somewhere, it's probably in the ATC Handbook, as exleardriver suggested.
 
MidlifeFlyer said:
No solid reference but I understand that the "trigger point" is 200' for altitude
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200' is my understanding also. Partly because that was what I was taught, and partly because I've never been contacted by ATC for anything less than 200'.
 
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