Re: Cleared for the approach and altitude

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On Tue, 01 Oct 2002 00:46:13 GMT, pstanley55@hotmail.com wrote:

>>
>>What ILS are you talking about where the cross bearing at this check
>>altitude comes from an NDB approach for the parallel runway. Please give
>>me the name/location of the approach so I can look at it.
>>
>cyyz, ils 24R

Well, I would use my DME + the GS to define NOAHA. Although not as
accurate as crossing a LOM, I would still expect the altitude to be within
30' of published and would feel that I had a problem if it was off by 100'.

>
>>I do a few approaches where the cross-bearing is a VOR radial, but, since
>>I've generally been vectored to the intercept altitude by ATC prior to
>>intercepting the GP, it's not been possible to figure an altitude error at
>>that intersection -- I look at the radial displacement instead from the
>>published VOR radial. That has always been within the published tolerances
>>for checking VOR's.
>>
>Not sure what you mean by "altitude error".

I mean the difference between the published altitude and my airplane's
indicated (or true) altitude.

>Also, do you mean when the check altitude is a vor radial , you check
>to see if the "check altitude" is within the tolerance of the vor?

Yes. For example, at CYYZ ILS 24R, if XRAY were a VOR (and not an NDB), I
would want to be at 2090' within +/- 2-3° of the charted radial.

>>I, personally, would not want to rely on an ADF cross-bearing to obtain a
>>check altitude, as my ADF is not all that accurate. However, I could try
>>to use my GPS to get an accurate crossing position -- just haven't been in
>>the circumstance where I'd need to.
>>
>>
>>>Now consider temperature effects. On a cold day, the check altitude
>>>can be out a few hundred more feet.
>>
>>Wow, do you fly IFR at those temperatures? Here in the east, I'd be
>>concerned about icing or, if it was much colder, we usually don't have low
>>IFR weather.
>>
>>In any event, it's easy enough to use an E6B to get the true altitude, if
>>the temperature is significantly different from standard. As a matter of
>>fact, it is this variability that is the reason for some stepdown fixes
>>that keep you above the GP (outside of the FAF) at a west coast airport (I
>>can't remember which one).
>>
>isa minus 44 is not so uncommon in the US. Minus 29 deg C in Minesota
>in winter, for eg.

That's true but, as I've said, I don't fly when it's that cold -- my heater
doesn't work so good. How often are you dealing with IFR weather when its
-29°C on the ground?

>A "check altitude" 1500 feet above the altimeter
>setting source gives you a 240 feet error. And as an aside, an
>intermediate segment gives as little as 451 feet obstacle clearance,
>so you may only have 211 feet true obstacle clearance.
>
>Have you ever heard of someone using E6B to correct for temp errors?
>

I have done that.

>I've never heard of instrument approaches taking into account the
>effect of lower true altitudes at cold temperatures. Can you provide
>anything further on this west coast airport. The TERPS does not take
>into account this temp effect, IIRC.

Actually, if I recall correctly, the problem comes about at that airport
with warm temperatures. The FAC passes over another airport. The top of
that airport's traffic area is defined by altimetry without temperature
correction. So on a warm day, the true altitude of the top of the ATA can
get higher than the GS altitude (or possibly so high as to not provide the
required clearance) (still outside the FAF). Stepdown altitudes are
provided so as to keep traffic on approach from infringing on this ATA.

It may not be in TERPS, but may have been added after consultation with the
local ATC facilities.

>.
>

>
>isa plus 22 (for you guys that don't fly where its cold) 1500 feet
>gives 120 foot error in the altimeter. Do you consider the
>temperature then when you decide 100 feet off would get you concerned?
>

I should! Again, here in the NE, those kinds of temperatures in
association with flyable IFR are unusual. I don't recall doing an ILS
approach in IMC with an OAT of 35°C.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)