Lateral limits of a block clearance?

SuperCubRick

SuperCubRick

Well-Known Member
I understand that I can ask for a block clearance from X to X radial between X and X miles that will give me room to maneuver as I wish.

However I'm not quite sure what the lateral boundries of a block clearance would be if I requested a block of altitude to fly along my route of flight. I'd imagine that so long as I stayed within the lateral boundries of the airway that I would be ok? What about if I'm /G and flying direct off-airway between two waypoints, what are the lateral boundries for that? I'd assume ATC expects me to remain within the 2 nm either side of my course as is displayed on the 430W operating in enroute mode.

Reason I ask is, if I see some clouds a few hundred feet below that I'd like to skim around using my block of altitude, how much lateral deviation would be acceptable?
 
A block clearance is an altitude clearance like any other. There is no lateral deviation built into one. You could however certianly ask to get a block altitude and direct with deviations say one or two miles to either side of your direct course.
 
Yeah - I guess I asked that wrong, lol. I guess the question is just asking what are the lateral limits of a GPS off-airway direct course or flying an airway with a GPS? I'm thinking 2 nm either side, but not sure.

If I'm navigating with GPS would they expect me to fly with less lateral deviation along an airway than if I were using VOR as primary nav? I'd assume they would - but couldn't technically give me the number to call unless I exceeded the 4nm lateral limits of the airway.
 
I don't know the technical book answer on RNAV direct deviation tolerence. Since none of my instrument time has used GPS I've never cared to look it up.

From my point of view as a controller, at the outer limits, VORs are shaky at best and I expect that. Just today I saw a DC9 tracking a STAR 40 miles from the VOR that was probably 5 miles off the track. I mean we're obviously talking about quality equipment and pilots so this just goes to show the limitations of a VOR. I wouldn't bat an eye if a GA pilot tracking a VOR was off course, I would tell them they aren't on the airway sure, but I wouldn't think that this pilot is purposly deviating.

From that same point of view RNAV is a different story. I'm expecting you to be pretty much dead on direct. Once I see that yes this pilot is direct, I'm not going to be anticipating a steep change in course. Say if you're on a 090 tracking an airway and I've got traffic 360 heading thats just passed you and stopped below you. once I have that divergence, I'm climbing that traffic. Now suddenly you whip it to 360 to stay IMC, I was relying on divergence, but now I don't even have that. I'm kind of sunk at that point. I know that is a very extreme example, but still.

I would have no problem at all if a pilot asked for a block and deviations. As a matter of fact, I'd love it if a pilot said they wanted a block and was going to need the full 4 miles to stay IMC. It keeps me informed of what you want to do, helps me help you stay IMC and keeps my sector running as smoothly as possible. Worst case senario is I terminate deviations until I have another form of seperation with the other traffic and you spend 90 seconds VMC.
 
i'm pretty sure the protected airspace for a direct RNAV route is 4NM either side of route. it's different for aircraft navigating off a NAVAID. as we all know the altitude and distance from the NAVAID dictates the protected airspace.

hope this helps.
 
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