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On Tue, 01 Oct 2002 06:09:44 -0400, Ron Rosenfeld
wrote:

>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.
>
You see tighter tolerances than I do. IIts only half the time that I
see less 100 feet difference. And again, theres temperature factor,
which I admit, I never calculate on a check altitude, and your the
first Ive error heard that takes this into account on a hot/cold
day>>
>>>I do a few approaches where the cross-bearing is a VOR radial, but, since
>>>Ive generally been vectored to the intercept altitude by ATC prior to
>>>intercepting the GP, its 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 VORs.
>>>
>>Not sure what you mean by "altitude error".
>
>I mean the difference between the published altitude and my airplanes
>indicated (or true) altitude.
>
ok, just again, Ive never heard anyone figuring an error in a gs
check altitude, thats all.
>>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 havent been in
>>>the circumstance where Id 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, Id be
>>>concerned about icing or, if it was much colder, we usually dont have low
>>>IFR weather.
>>>
>>>In any event, its 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
>>>cant remember which one).
>>>
>>isa minus 44 is not so uncommon in the US. Minus 29 deg C in Minesota
>>in winter, for eg.
>
>Thats true but, as Ive said, I dont fly when its that cold -- my heater
>doesnt 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.
>
>>Ive 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 airports 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 dont 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 dont recall doing an ILS
>approach in IMC with an OAT of 35°C.
not in the NE, but certainly in the SW
Stan>
>
>Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

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