Why can't I use a METAR for MDA?
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Why can't I use a METAR for MDA?
I notice that the latest amendment to AIP precludes me from using METAR QNH EVER for instrument approaches (it isn't listed in ENR 1.5 5.3.1, and it's not an approved source).
Could one of the boffins please explain the rationale behind this change?
Given that the forecast QNH (whether it be Area or TAF) can be in error by some millibars compared to a Metar, this change will have significant negative ramifications on the actual height of the aircraft at the MDA, with the aircraft possibly being much higher than it would've been had a more accurate QNH (such as a Metar) been used (I am not talking about reducing the MDA by the 100ft either).
Obviously, a Metar QNH (even given that it cannot be used for the 100ft reduction) is a far more accurate QNH than either of the Forecast QNHs, and we should be allowed to use it for Instrument Approaches. Why can't we?
Could one of the boffins please explain the rationale behind this change?
Given that the forecast QNH (whether it be Area or TAF) can be in error by some millibars compared to a Metar, this change will have significant negative ramifications on the actual height of the aircraft at the MDA, with the aircraft possibly being much higher than it would've been had a more accurate QNH (such as a Metar) been used (I am not talking about reducing the MDA by the 100ft either).
Obviously, a Metar QNH (even given that it cannot be used for the 100ft reduction) is a far more accurate QNH than either of the Forecast QNHs, and we should be allowed to use it for Instrument Approaches. Why can't we?
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You can use QNH derived from a Metar/Speci for your instrument approach. However you can not use a Metar/Speci derived QNH to take advantage of the 100ft actual QNH reduction on the approach plates that allow it.
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Bargy,
But Metars are not listed in the 3 types of QNHs "am required" to set "prior to passing the IAF" (ENR 5.3.1). That is my point. Since Metars are NOT an approved source (5.3.2), then I can't use it at all because it's not listed. Perhaps the rule was drafted poorly, and the intent was to allow us to use Metar QNH, but that is certainly not what it currently says.
Without being too pushy, what rule do you reckon allows us to use the Metar QNH?
Of course, in the old days (prior to 17Apr) there were no rules (except the sources that constituted "an approved source" and the -100ft business).
But Metars are not listed in the 3 types of QNHs "am required" to set "prior to passing the IAF" (ENR 5.3.1). That is my point. Since Metars are NOT an approved source (5.3.2), then I can't use it at all because it's not listed. Perhaps the rule was drafted poorly, and the intent was to allow us to use Metar QNH, but that is certainly not what it currently says.
Without being too pushy, what rule do you reckon allows us to use the Metar QNH?
Of course, in the old days (prior to 17Apr) there were no rules (except the sources that constituted "an approved source" and the -100ft business).
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Got to work and saw the amendment you are talking about waiting in my pidgeon hole.
As far as I can tell nothing has changed.
Remember, a metar is a routine aerodrome weather report by an approved observer or an aws.
Part a) of the amendment would cover these type of reports.
I think that in expanding this clause they are just seeking to cover their ar$es as usual.
Cheers.
As far as I can tell nothing has changed.
Remember, a metar is a routine aerodrome weather report by an approved observer or an aws.
Part a) of the amendment would cover these type of reports.
I think that in expanding this clause they are just seeking to cover their ar$es as usual.
Cheers.
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Not sure of the applicable amendment you are referring with AIP as i use Jepps.
You have never been able to use QNH source from a METAR for the extra 100 ft on mda's.
However In Jepp Terminal 5.3 it advises on what can be used to reduce the mda by 100ft. Briefly the sources are ATC, ATIS, AWIB and CASA approved met observers. Additionally in Jepp Meteorology 4.3.6 it also advises that QNH derived from AWS is an acceptable source of actual QNH for reducing MDA. The QNH is valid for 15 mins from the time of receipt.
It is nonetheless confusing as an AWS observation appears with the title METAR
You have never been able to use QNH source from a METAR for the extra 100 ft on mda's.
However In Jepp Terminal 5.3 it advises on what can be used to reduce the mda by 100ft. Briefly the sources are ATC, ATIS, AWIB and CASA approved met observers. Additionally in Jepp Meteorology 4.3.6 it also advises that QNH derived from AWS is an acceptable source of actual QNH for reducing MDA. The QNH is valid for 15 mins from the time of receipt.
It is nonetheless confusing as an AWS observation appears with the title METAR
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Howdy Farcome.
The latest Jepp, 25 April and AIP amendment covers CCs' question.
There is no change to the "actual QNH" requirement for the 100' minima reduction on an approach. It's just a separate clause now.
Your reference to AWS sources for this 100' reduction applies to an AWIB. (ie) broadcasts of actual weather conditions.
I think what CC is asking is refering specifically to a metar and the use of it under the new amendment, for a QNH source to set at the IAF of an approach(not for the 100' reduction).
Metars can be derived from either an approved observer or an AWS so they can be used for this purpose, as i said before, under part a) of Jepp Terminal clause 5.3.1 (same for AIP)
Hope I'm not giving a 8um steer with all this but is sounds right to me.
Barge
The latest Jepp, 25 April and AIP amendment covers CCs' question.
There is no change to the "actual QNH" requirement for the 100' minima reduction on an approach. It's just a separate clause now.
Your reference to AWS sources for this 100' reduction applies to an AWIB. (ie) broadcasts of actual weather conditions.
I think what CC is asking is refering specifically to a metar and the use of it under the new amendment, for a QNH source to set at the IAF of an approach(not for the 100' reduction).
Metars can be derived from either an approved observer or an AWS so they can be used for this purpose, as i said before, under part a) of Jepp Terminal clause 5.3.1 (same for AIP)
Hope I'm not giving a 8um steer with all this but is sounds right to me.
Barge
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Hi guys,
My first post.
I figure the answer is this....3 hpa equals 90 odd feet. 4 equals
120 feet.
The obstacle clearance from the MAPT provides only 100' clearance along the missed approach path. Therefore on a single engine missed approach from the minima with your QNH out by 4 hpa (quite possible) your strawberry jam spread on someones hillside. Thats why you never go below minima
No proof on that one ..just common dog fark
My first post.
I figure the answer is this....3 hpa equals 90 odd feet. 4 equals
120 feet.
The obstacle clearance from the MAPT provides only 100' clearance along the missed approach path. Therefore on a single engine missed approach from the minima with your QNH out by 4 hpa (quite possible) your strawberry jam spread on someones hillside. Thats why you never go below minima
No proof on that one ..just common dog fark
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Captain Custard - Unless you have the METAR read out to you prior to starting the approach, I don't understand how you would use a METAR QNH anyway. I use the RAAFs Flight Info Handbook vice AIP, and I've never used a METAR for anything other than checking the actual conditions before stepping for a flight. I guess if a METAR was given to you just after it was produced, you could use the QNH for 15 minutes (the validity of an approved source QNH) as the METAR was presumably produced by an approved MET observer.
DrP.
DrP.
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Dr P,
There's nothing wrong with using a METAR QNH for an approach. You just can't use it for the 100' reduction to minimas.
Terminal 5.3 in the Jepps also clearly states that you can't use METAR QNH for the purposes of this minima reduction, regardless of the fact that it may be less than 15 minutes old. It's still a METAR isn't it.
If an approved observer gave you the conditions over the radio or as you said, as you were stepping for a flight, then you would be receiving an observation of actual conditions, the same as an awib. But not a METAR.
There's nothing wrong with using a METAR QNH for an approach. You just can't use it for the 100' reduction to minimas.
Terminal 5.3 in the Jepps also clearly states that you can't use METAR QNH for the purposes of this minima reduction, regardless of the fact that it may be less than 15 minutes old. It's still a METAR isn't it.
If an approved observer gave you the conditions over the radio or as you said, as you were stepping for a flight, then you would be receiving an observation of actual conditions, the same as an awib. But not a METAR.
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Doc and all,
The issue is not whether we can use a Metar to reduce the MDA by the 100ft: that is not allowed. My enquiry is whether we can use a Metar at all.
A typical scenario is this: 30 minutes from the destination we get the latest Metar from Flightwatch (which may be up to 30 minutes old). We then set this on the altimeters and do our instrument approach (not reducing the MDA).
Under the new rules, is this legal?
And if so, how old can the Metar be before we can't use it?
Is this why CASA created the rule (so that any old Metar can't be used?)? If so, and given that a Metar (by definition coming from a BOM observer) would most probably be more accurate than the airfield TAF, why can't we use a Metar if it was, say, up to 2 hours old? After all, the TAF only has 3 hourly QNHs on it.
The issue is not whether we can use a Metar to reduce the MDA by the 100ft: that is not allowed. My enquiry is whether we can use a Metar at all.
A typical scenario is this: 30 minutes from the destination we get the latest Metar from Flightwatch (which may be up to 30 minutes old). We then set this on the altimeters and do our instrument approach (not reducing the MDA).
Under the new rules, is this legal?
And if so, how old can the Metar be before we can't use it?
Is this why CASA created the rule (so that any old Metar can't be used?)? If so, and given that a Metar (by definition coming from a BOM observer) would most probably be more accurate than the airfield TAF, why can't we use a Metar if it was, say, up to 2 hours old? After all, the TAF only has 3 hourly QNHs on it.
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Jepp Terminal 5.3.1 says:
Prior to passing the IAF, pilots are required to set either:
a)the actual aerodrome QNH from an approved source, or
b)the forcast Terminal QNH, or
c)the forecast area QNH.
A METAR is a routine report by an approved observer or an AWS of actual conditions.
Thus it comes under part a) of above clause.
You can't use it for the lower minimas because it doesn't meet the validity requirements that other actual reports do, ie awib, atis etc. ]
METARS are published every 1/2 hour and hour depending, therefore they don't give the actual conditions say 10 mins after being published.
Prior to passing the IAF, pilots are required to set either:
a)the actual aerodrome QNH from an approved source, or
b)the forcast Terminal QNH, or
c)the forecast area QNH.
A METAR is a routine report by an approved observer or an AWS of actual conditions.
Thus it comes under part a) of above clause.
You can't use it for the lower minimas because it doesn't meet the validity requirements that other actual reports do, ie awib, atis etc. ]
METARS are published every 1/2 hour and hour depending, therefore they don't give the actual conditions say 10 mins after being published.
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Otto,
But a Metar (or the QNH bit, at least) is excluded from the list of approved sources of actual QNH as listed in 5.3.2. That is my point.
The note confirms that the QNH from a Metar cannot be used for 5.3.1a (or at least it implies that). Now whether that was what CASA intended or not, that's what I'm asking. Remember that the amendment simply inserted para 5.3.1 into the section: perhaps the note (or it's implications) was forgotten about.
A case of the left hand not telling the right hand what's changing (although that is a bit hard to understand given the amended bit is right above the note)?
But a Metar (or the QNH bit, at least) is excluded from the list of approved sources of actual QNH as listed in 5.3.2. That is my point.
The note confirms that the QNH from a Metar cannot be used for 5.3.1a (or at least it implies that). Now whether that was what CASA intended or not, that's what I'm asking. Remember that the amendment simply inserted para 5.3.1 into the section: perhaps the note (or it's implications) was forgotten about.
A case of the left hand not telling the right hand what's changing (although that is a bit hard to understand given the amended bit is right above the note)?
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CC.
5.3.2 is a completely separate clause and has nothing to do with 5.3.1.
5.3.2 relates only to telling us what the approved sources are for reducing the minima by 100' on the approach.
The note that says METARs do not meet the requirement for actual QNH also only relates to clause 5.3.2 as that is the clause it is noted in.
5.3.2 is a completely separate clause and has nothing to do with 5.3.1.
5.3.2 relates only to telling us what the approved sources are for reducing the minima by 100' on the approach.
The note that says METARs do not meet the requirement for actual QNH also only relates to clause 5.3.2 as that is the clause it is noted in.
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While we are on approaches, in the latest Jepp amendment, 25 APR 03, the terminal section under ILS has changed.
Para 2.7.3.1 used to be the one that stated that at the altitude check if you were high you added that on to the DA/H.
It now states:
"The final approach segment contains a fix (FAP) at which the glide path/altimeter relationship should be verified. If the check indicates an unexplained discrepancy, the ILS approach should be discontinued. The following should also be complied with:" etc,etc. Part c of this now states "For operations where the temperature on the ground is colder than -15 degrees C, the DA/H should be adjusted to true altitude."
Does this mean if you show 50' high and you consider that an explained discrepancy, you continue to the published DA/H unless it is colder than -15, or has the requirement to add the altitude been moved somewhere else in Jepp's?
Para 2.7.3.1 used to be the one that stated that at the altitude check if you were high you added that on to the DA/H.
It now states:
"The final approach segment contains a fix (FAP) at which the glide path/altimeter relationship should be verified. If the check indicates an unexplained discrepancy, the ILS approach should be discontinued. The following should also be complied with:" etc,etc. Part c of this now states "For operations where the temperature on the ground is colder than -15 degrees C, the DA/H should be adjusted to true altitude."
Does this mean if you show 50' high and you consider that an explained discrepancy, you continue to the published DA/H unless it is colder than -15, or has the requirement to add the altitude been moved somewhere else in Jepp's?
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Otto,
While we're on the subject of QNH, do the Jeppo pubs give any further guidance when using Area QNH for an approach? The latest RAAF Flight Info Handbook now requires us to add 50' to the MDA when using area QNH.
DrP.
While we're on the subject of QNH, do the Jeppo pubs give any further guidance when using Area QNH for an approach? The latest RAAF Flight Info Handbook now requires us to add 50' to the MDA when using area QNH.
DrP.
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QNH OK, ILS "What the"
Hi All
I think the QNH source thing has been well answered now. But I still haven't found the add 50ft if AREA QNH used. Can someone post a reference. (jepps please)
As for the ILS, there was a good discussion on this at work the other day.
The add to the minima clause if indicating high at the OM check height appears to be gone.
The wording of the new paragraph says, "unexplained discrepancy". to me this means if you can account for the discrepancy then you don't need to "discontiue the approach".
ie. You are slightly high on the GS, Yea I know that NEVER happens, and the alti check is slightly high. well the discrepancy is explained !.
I think the following questions are also valid for the ILS,
1. "Discontinue the approach", well now do you conduct the Missed approach or change to the Localizer ?, any ideas.
2. How far do we stretch, or when does an "Unexplained Discrepincy", become an explainable one.
PS, unless it has changed, and I must say I have not looked, the AIP/Jepps do say that the company is responsible for decaling the minimums to be used given performace and procedure considerations for aircraft and approaches. Sort of saying that, here are the absolute minimus, now you work out what you want to add for your operation. Oh and by the way if you prang, its your fault entirely.
I think the QNH source thing has been well answered now. But I still haven't found the add 50ft if AREA QNH used. Can someone post a reference. (jepps please)
As for the ILS, there was a good discussion on this at work the other day.
The add to the minima clause if indicating high at the OM check height appears to be gone.
The wording of the new paragraph says, "unexplained discrepancy". to me this means if you can account for the discrepancy then you don't need to "discontiue the approach".
ie. You are slightly high on the GS, Yea I know that NEVER happens, and the alti check is slightly high. well the discrepancy is explained !.
I think the following questions are also valid for the ILS,
1. "Discontinue the approach", well now do you conduct the Missed approach or change to the Localizer ?, any ideas.
2. How far do we stretch, or when does an "Unexplained Discrepincy", become an explainable one.
PS, unless it has changed, and I must say I have not looked, the AIP/Jepps do say that the company is responsible for decaling the minimums to be used given performace and procedure considerations for aircraft and approaches. Sort of saying that, here are the absolute minimus, now you work out what you want to add for your operation. Oh and by the way if you prang, its your fault entirely.
