Instrument Approach Deviation

[Jmmccutc]

Just got in a discussion a few minutes ago and am trying to get a question resolved before i give our cheif pilot an anurism from yelling...

Question: Once established on an approach (VOR in particular) how far can you "stray" and come-back on course with out having to go missed (how far can the needle travel?)

My answer: 9.9999999 degrees, as long as it doesn't go 10 degrees, full deflection, you're cool...Now i'm not saying i'm right, i just have that in the back of my head and don't have any books to look it up currently...

His Answer: You have to keep the needle nailed dead center or you can't inniate a step down due because you're not sure where the obstacles are located off of the center line...

i can see where he's goin, but i don't think that's right, i know there is a given ammount of lee-way just not sure how much...and like i also said i don't think that i'm correct but i don't think he is either...

 

[Andy Neill]

Practical Test Standards for the Instrument rating allow a 3/4 scale deflection on the OBS or 10 degrees when using an RMI. Seems like that would not be unsafe.

+/- 0 is the pro's objective but less than that level of performance is still within safety margins.

TERPS would give you the precise leeway involved.

 

[Jmmccutc]

Practical Test Standards for the Instrument rating allow a 3/4 scale deflection on the OBS or 10 degrees when using an RMI. Seems like that would not be unsafe.

Andy, Thanks, i had already printed out the PTS to support my argument, i think it helped...i figured if the pts allow you 3/4 deflection about 7.5 degrees, you'll be about 1.25 NM off at 10 miles...i don't know of many VOR approaches that are longer than 20 so at most you'll be 2.5 NM off course...this to me didn't seem like an unrealistic amout of area for a "safe zone" to gaurentee obstacle clearence...

thanks again...

 

[Tinstaafl]

The PTS's are all about what you need to do to pass a test. They're not documents that specify tolerance exceedances that mandate a missed approach, although it's part of what a procedure designer has to consider when designing an approach.

What if you hold an ATP? The tolerance becomes 1/4 scale deflection to pass the test. Using PTS standards as the missed approach requirement would mean that a PPL/CPL with an IR must commence the missed once 3/4 scale is breached whereas an ATP doing exactly the same approach has to do the missed once 1/4 scale is broached.

Some countries specify a missed approach tolerance. Australia used to specify half-scale/5 deg, which also happened to be the IR tolerance.

 

[Bluto]

I don't think they were suggesting the PTS is the regulatory limit. Whether you hold an ATP or a PPL is irrelevant. If the PTS allows 3/4 deflection, then clearly that is acceptable to the FAA. As such, it would be difficult to argue that zero deflection is allowed.

 

[A Squared]

A couple of things to think about:

1) You are allowed to fly IFR with a 4 degree error in your VOR (6 with an airborne check) That's 2 dots on the CDI. Why aren't people flying into things all the time?

2) When you fly over a VOR, there is always almost a period of time when the needle is at full deflection.

Of course there are tolerences built into the system. the notion that there isn't is just absurd.

 

[SDCFI]

His Answer: You have to keep the needle nailed dead center or you can't inniate a step down due because you're not sure where the obstacles are located off of the center line...

Huh? When do you descend from the PT altitude to the FAF alt after a PT? When the needle comes off the peg and you are "established" on the inbound course. So I would say you would execute the missed with a full-scale deflection because you are no longer established, and you don't know how far off you are (could be 1/2 mi, could be 5 miles.) To imply that you might hit something with anything except a dead center approach is quite ignorant in my book. Shooting for perfection is one thing, but I haven't met anyone yet who is perfect. And like A Squared said there are allowable errors for VOR checks, so even with the needle centered you may be 4 or 6 degrees off course. Does this mean you might hit something on evey approach?

 

[User546]

His Answer: You have to keep the needle nailed dead center or you can't inniate a step down due because you're not sure where the obstacles are located off of the center line.

I look at it logically like this:

If the FAA says that you can continue an approach until full deflection (at which point you must go missed), then the tolerances ARE built into these approaches that gives you a protected area, up to the width of a full scale deflection.

They're not going to design an approach that will take you into a mountain when you go 3/4 scale deflection! And I've flown with enough people in the day that makes that a really good idea!!

Same as turning inbound on a procedure turn. Your not allowed to make your step down until your established on the published course. I was taught (and its logical) that you're established on the course, once the needle comes alive on the inbound intercept. This applys to your scenario just as well. If the needle is alive, your going to be in protected "airspace".

 

[A Squared]

Let's throw out some numbers, just to see how this really fits toghther.

On a VOR approach with a FAF, the *narrowest* part of the final approach segment will be 2 miles wide. now, if you're a dot off center, at 5 miles from the VOR, you'll be 0.175 NM from the centerline. Of course, at 5 miles the protected area is 2.25 nm wide.

What if you're right at full scale deflection? At 5 nm from the VOR that would put you 0.88 nm from course centerline, which is starting to get close to the edge.

This is for a VOR approach with a FAF. If there's no FAF (VOR on the field) the protected area is much wider.

If you're on the intermedaite segment of the approach, the protected area is much wider.

 

[HMR]

My answer: 9.9999999 degrees, as long as it doesn't go 10 degrees, full deflection, you're cool...

You're right. I had a custom made HSI put in our plane that has an extra dot on each side (12 degrees instead of 10). We go missed 10% less than before.

 

[rvsm410]

Localizer obstruction clearance, Plus MSA

ok reference the AIM 2004, PG 433 PARA 5

The Localizer provide course guidance throught the decent path to the rnwy threshhold from a distance of 18nm fm the antenna between an atlitude of 1000' above the highest terrain along the course line and 4500' above the elevation of the antenna site.

the MSA on a plate provides 1000' clearance above the highest obstical ina 25 nm radius of the facility, so when intercepting the glide slope (up to 20 nm out) you will be guaranteed obstical clearance so long as you dont go Below the GS...

 

[User546]

You're right. I had a custom made HSI put in our plane that has an extra dot on each side (12 degrees instead of 10). We go missed 10% less than before.

I have to ask, why would you have an HSI custom made to have an extra dot?

 

[A Squared]

I have to ask, why would you have an HSI custom made to have an extra dot?

So he can go missed 10% less. I'm thinking you never saw "this is Spinal Tap"

 

[HMR]

I'm thinking you never saw "this is Spinal Tap"

.....

 

[TrafficInSight]

These go up to eleven.

 

[FlyingBuckI]

I was always told that course intercept should be 2 degrees (one dot) or less deviation to descend, so that's what I've always done (and taught)...

 

[Tinstaafl]

In Oz & the UK 'established' is defined as half-scale deflection for VOR or LOC & 5 deg for NDB (which gives equivalent accuracy to VOR). Needle alive isn't accepted.

A question for you: If needle alive is the specification for VOR &/or LOC under TERPS, what is it for NDB?

 

[A Squared]

A question for you: If needle alive is the specification for VOR &/or LOC under TERPS,

It isn't. the TERPS specifies approach design criteria. It does not specify standards or procedures for flying. As far as I know, there is no official document when tells you whether you have to have one dot, 2 half scale or full deflection to descend. This is a fairly common debate, and I don'k know of any official guidance.

what is it for NDB?

The same, no official guidance. The most common criteria I've heard is 10 degrees, but that's not official.


Here's what the TERP does say:

When there is no FAF (VOR or NDB is on the field), that is the case with the narrowest protected area at the point where you're joining the inbound course from the procedure turn. In such an approach, the *primary* protected area is at least 16 degrees either side of the course centerline. (there's a secondary area when extends this protection with a tapering altitude margin) So needle alive, or 10 degrees on the ADF would put you within the primary area. Of course, half scale, or 5 degrees would be even more conservative. ONe dot from center would be *very* conservative.

In the case where the facility is the FAF, the protection is even wider, 21 degrees, plus a secondary protection area.

 

[wxman13]

The other thing to keep in mind is that if there are obstacles (antennas, tall buildings, whatever) on or near the final approach path, they will appear ON THE PLATE as little symbols with elevations. Likewise, if you're using NOS charts, relevent topo lines and elevations will be charted. So it's not like you're flying blind and unaware of signifcant obstacles that will snag you if you drift .1 miles laterally off the centerline. I don't know for sure, but from everything I've read, approaches are designed to be really conservative--think worst case scenario for the pilot, flying either partial panel or getting no-gyro vectors, and maybe with smoke in the cockpit or more realistically, an old, crappy CDI that likes to jump around.

Peter

 

[nosehair]

This amazes me every time I see this about instrument approaches. It's actually scary to know that there are so many pilots flying on hearsay. "I was taught to do so and so..."

Every instrument pilot should know enough about TERPs to have a general idea about how much airspace is required to set up an instrument approach or a holding pattern. It is very complicated and too much to carry around specific knowledge because there are so many variables....but after some study, any pilot will realize that he has way more room for error than any training program allows....and let's be thankful for that. Which is probably the reason it is not taught...then the sloppy pilot will push the limits.

PTS standards are not TERPs. Will you hit terrain if you are 200 feet off altitude? No, but you might bust a checkride. ATC won't even mind if you are only 200 feet off. 300 is where they get concerned, and could bust you if you are on an IFR asigned altitude, but hey, you probably know you really have a thousand feet before you actually hit something.

Wanna make 30 degree left and right turns chasing the localizer needle? You can if it is a private flight, as long as the needle is within the pegs.

Even if the needle goes full scale, you don't HAFTA make a missed approach, you are not gonna hit something the second the needle hits the peg. If you see it peg out and are correcting, you are still safe. How far you can drift beyond full scale deflection is an area of concern. If I get real busy and don't watch my needle for maybe 10 or 15 seconds (which ain't gonna happen) (ok, it could) then I suddenly discover it is full scale, and I don't know how long, well, then I would probably start a climb while turning towards the needle, and proceed from there. If I can recapture the needle before going too high...

The point is that every instrument pilot should know the limits set out by TERPs and fly accordingly. Making a missed approach is not a save-all decision. A missed approach can develop into many other problems*fuel*weather*etc.