Min altitude during a procedure turn

[Singlecoil]

That's all well and good for standard altimeters, but if memory serves, the air data computers in our cockpit correct for this on our digital altimeters...

If I'm mistaken, someone need only point it out to me...

You're wrong. No ADC's correct for non-standard temps. Common misconception. Think about it. There would be no way for ATC to separate traffic if some aircraft were correcting and some weren't. A 737 passing a 402 with 1000 foot separation? They all have to have the same reference datum for ATC separation to work.

 

[Whirlwind]

You're wrong. No ADC's correct for non-standard temps. Common misconception. Think about it. There would be no way for ATC to separate traffic if some aircraft were correcting and some weren't. A 737 passing a 402 with 1000 foot separation? They all have to have the same reference datum for ATC separation to work.

That makes sense... Oh well, another theory bites the dust...

Yet another reason why I'm an eternal student of aviation...

 

[A Squared]

They all have to have the same reference datum for ATC separation to work.

They *would* have the same reference datum (mean sea level) they would just be measuring thier height above the datum differently.....I know, I know, semantics.

>>>>On a cold day you could be lower than indicated by 500 feet or more.

For example, If you were going into Petersburg, PT alt. 7800 ft, and you were looking at -20c OAT, you could expect to be about 600 ft lower than your indicated altitude.


TO address the original question by whirlwind: Look at the PTS's ... yeah I know you aren't asking about the PTS, but bear with me ....
Both the instrument rating and the ATP PTSs allow +/- 100 feet of altitude on an approach before the final approach segment.

Now think about it...would the PTS allow a level of error on a check ride which would constitude a regulation violation if it wasn't a checkride?

To put it differently, if it is a bust to go 20 feet below PT altitude, why are you allowed to go *100* feet below it on an ATP checkride?

Doesn't make sense, does it. I think that we can conclude that you are allowed at least 100 ft below the altitude .. how much, I don't know. ATC considers 300 feet off an assigned altitude to be a bust right? I wouldn't wander 300 ft below the PT altitude at Petersburg on a cold day though.

 

[philo beddoe]

Several more points here:

First, I was giving a very general overview of the PT protected area. You are correct, the TERPS gives a diagram of it, also showing the secondary protected area (2nm wide).

Essentially, if you are at the correct PT altitude (assuming no altimeter or temp errors), you would need to be 6nm off course on the non-protected side, 8nm off course abeam the fix on the PT side, 10nm off course at the (normally) 10nm outbound limit, or fly a whopping 18 nm outbound before you are outside of all protected areas (primary and secondary).

(A pic is worth 100 words, is suppose, but someone with more flightinfo skill than me can post the diagram.)



Also, temp errors can be considered, but the most likely place for real peril to occur would be a medium to high PT in mountainous terrain, with very cold temperatures.

The cold-wx effect on DH or lower MDA is mostly negligible in all but the coldest of temperatures.

This is because the cold-wx error compounds with altitude above the altimeter reporting station, not your height MSL.

Essentially, no matter how cold it gets over the flat midwest, you are unlikely to be enough lower to have a real problem.

That said, the compounding effects of cold temps, altimeter mechanical error and pilot error could make a 'perfect storm' situation that could put you much closer to the trees than you like.


Take a typical example:

VOR approach with a published FAF: 250ft of clearance minimum above the controlling obstacle.

minus 75' altimter error. = 175'

minus 50' cold wx error = 125'

minus sloppy piloting 50' = 75'

Now, the likelyhood that this combination of events will occur at the exact instant you are overthe controlling obstacle is unlikely.

No one though you could lose all the hydraulics in a DC-10, either.

 

[avbug]

They all have to have the same reference datum for ATC separation to work.

They do, and that would be 29.92, as all encoders work on pressure altitude. Altimeters do not compensate for temperature.

How far off can you be on an approach? What kind of question is that? How about something more appropriate, like how accurate can you be?

How much below published minimums can you fly? Minimums minus zero...and you can certainly be above them. Why would you permit yourself to go below a minimum at all?

Enroute, you have much greater terrain and obstacle protection than on an approach. You are flying lower, until at circling minimums within the circling radius for your category, you're down to 300 feet of obstacle protection.

Considering the previous conversation herein, where a 75' altimeter allowance must already be presumed to exist (it can), other altimeter errors may seriously erode your protection. Lateral protection can be tight, too, despite TERPs general criteria. Even so far as not permitting full deflection on the CDI due to terrain (Flagstaff AZ, and Ketchikan AK spring to mind). Add to that the errors induced by a temperature change, and I can see no reason why you would ever want to permit descent below a published minimum altitude.

If you're given minimum altitudes during a proceedure turn and subsequent inbound descent to the airport, adhere to them. Don't seek out loopholes or excuses to deviate below them. If you're going to err, then do so above them, but not below.

I don't know about you, but I consider any minimum altitude published on an instrument proceedure to be a hard deck; a solid floor I'm not willing to violate.

How much altitude are you allowed legally to deviate below a minimum published on an approach? None. How much will a check airman or inspector or examiner allow you to get away with during a practical test? Ask that person. But that person may violate you the moment you go below the minimum, if he or she so chooses...so don't do it. There's no reason for it.

 

[A Squared]

AVbug (and others) perhaps some clarification is in order. I was in no way advocating descending below any altitude, to any degree, although I suppose that it could be read that way.


I was answering the question which was asked.

*If* the question was "how far below PT altitude should you consider acceptable?" The answer is: "do not go below the altitude, if for any reason you find yourself below that altitude correct immediately"

That was *not* the question however, the question was "how far below the altitude constitutes a violation of the regulations" That is a completely different question, and that is the one I was answering.

 

[100LL... Again!]

Avbug - not necessarily so regarding checkrides - a minus 20' deviation on the PT is not grouds for a fail.

I agree that mins are to be treated like a hard deck, but if I remember right the pt and pt inbound to faf segments are 100' either way.

mda is plus 50, minus 0 (?) I'll look it up later

examiners CAN try to enforce tighter standards, but they should not be using their own "personal PTS" whether less or more strict.

 

[minitour]

100LL
You're mostly right

Instrument Rating PTS
VI. Area of Operation: Instrument Approach Procedures:
bla bla bla

A. Task: Nonprecision Approach

bla bla bla

Objective. To determine that the applicant:

9. Mainains, prior to beginning the final approach segment, altitude within +/- 100 feet, heading within +/- 10 degrees and allows less than 3/4 scale deflection of the CDI or within +/- 10 degrees in the case of an RMI, and maintains airspeed within +/- 10 knots.

13. Maintains the MDA, when reached, within +100 feet, -0 feet to the MAP

B. Task: Precision Approach

Objective. To determine that the applicant:

8. Mainains, prior to beginning the final approach segment, altitude within +/- 100 feet, the desired airspeed within +/- 10 knots, the desired heading within +/- 10 degrees; and accurately tracks radials, courses, and bearings.

Not sure what the ATP PTS looks like, but for the IR ride before the FAF everything is +/-100 feet.

That said, if there is no FAF and you're in the procedure turn, I'd hope you aren't below the PT altitude since once you're inbound you're on the Final Approach Segment.

Scratch that...once you're inbound you're allowed to descend to MDA anyway (unless you've got some stepdown fixes) so as long as you don't blow through the MDA you should be fine.

-mini

 

[potrack]

Phillo Beddoe is right on when he discussed the ROC in the primary area is 1000 feet.

Caution, the secondary area, whick i believe is a 2nm buffer around the primary, tapers from 1000' against the primary out to ZERO obstacle clnc at its outer edge.

 

[A Squared]

Phillo Beddoe is right on when he discussed the ROC in the primary area is 1000 feet.

Caution, the secondary area, whick i believe is a 2nm buffer around the primary, tapers from 1000' against the primary out to ZERO obstacle clnc at its outer edge.

Good, point, howeever it's even lessthan you say, the seconary area tapers from 500' of clearence to zero at the outer edge