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nunans

Altimeter with no subscale??

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Hi, I purchased a plane recently (supapup MkII) and the altimeter has an adjustment knob but no QNH subscale. It's the first one i've seen that doesn't have a subscale and i'm not sure what to think,

 

Does it really matter?

 

I guess visual flying is what ultralights do so even the altimiter itself is optional?

 

There is no stall warning either so i'm beginning to realise recreational aircraft can require more hands on skill to fly than the GA trainers with all the warnings and instruments and inherent stability built in.

 

Scott

 

 

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do u need to use QNH Scott?....if not u dont need a subscale..

I'm new to all this but I suppose the only time i'd need to use QNH is to keep my altimeter accurate en route to stay hemispherical and out of the way of other aircraft, or when inbound to set local qnh with an awis/atis etc so i end up at the right circuit height.

 

In reality i probably won't be going far so just to look out the front and stay above the dirt should be good enough.

 

Maybe once i get some hours up i won't need an altimeter at all?

 

 

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Scott has a knob to adjust it - no need to know what the subscale setting is to use the knob.

 

 

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That's how I understand it. So knowing the elevation of my location on the ground I would adjust my altimeter to that elevation which would be QFE by default even though you could not actually read the QFE without a subscale.David

If you know the field elevation and set that while on the ground then you are setting QNH if you are on the ground and set ZERO on the alt you are setting QFE.

 

 

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Tex, that's it, pure and simple. QFE is simpler to use if you are ONLY doing circuits at the ONE aerodrome. It's not ( encouraged to be ) used in Australia.

 

An altimeter without a Kollsman scale is a fairly useless instrument. So is an altimeter that isn't calibrated correctly. Remember, we fly pressure heights, not actual heights ( as would be available from a GPS). An altimeter is calibrated with reference to the ICAO standard atmosphere, and they should all be the same. IF they are out of calibration all your separation by using heispherical heights is compromised. Nev

 

 

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If you know the field elevation and set that while on the ground then you are setting QNH

Actually, that would be Local QNH, not AREA QNH, which is the QNH that everyone with an altimeter with a subscale would be using.

 

If I was flying around Oberon, I'd want an accurate altimeter.

 

OME

 

 

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and Local QNH is good for a 5 mile radius...but.....not much else.

033_scratching_head.gif.b541836ec2811b6655a8e435f4c1b53a.gif Where is that from CFI?

 

Local QNH is generally sufficient and you can not expect to get an accurate area QNH (quite different to the airfield you are sitting at) given that it is a forecast average with a validity period over a big area and not particularity relevant to many localities that we tend to fly (perhaps outside the J curve). If you are using the AWIS then you are getting the local QNH sub scale setting anyway. So if someone is on an area QNH from an ATIS and someone is on the AWIS local QNH they could be cruising at different levels... but they are both valid in RAA ops outside CTA.

 

Moot for nunans purposes anyway with no sub scale... local QNH is sufficient, perhaps not acceptable to some but plenty sufficient.

 

QFE...is only good if u have a certain accent.....

I don't get it blink.gif.7ee21b69ed31ab2b1903acc52ec4cc3f.gif 056_headset.gif.8e2503279a37389023f4d903d46b667a.gif

 

 

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Tex, unless there was a large pressure gradient or a deep system moving rapidly the error would not be a large one. You are probably never more accurate than when you set YOUR calibrated and TSO'd altimeter to the actual aerodrome elevation, do circuits and reset, or check it every couple of circuits. 1 mb = 30 feet, so 1 or 2 is not much. There is an allowable tolerance for an altimeter to be considered serviceable/unserviceable. Nev

 

 

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Exactly Nev 012_thumb_up.gif.cb3bc51429685855e5e23c55d661406e.gif

 

There is an allowable tolerance for an altimeter to be considered serviceable/unserviceable. Nev

100' I believe.

 

 

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Think it is around that. Think it varies depending on the altitude where you make the comparison. We can only fly our own plane, much of what happens is up to the other guys/ gals. Keep a good lookout people. Nev

 

 

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As i understand it, it depends on the type of transponder.... you need Mode C for CTA ops. If you are requesting a clearance for controlled airspace you will have to identify the ATIS and that also means setting the area QNH. I thought when set to alt the Mode C transponder displayed the alt from your altimeter (or encoder) as set by you, not just to 1013.2. If you are outside controlled airspace or Class E running a squawk on 1200 the same will occur (depending on transponder and alt setting), and they advise any 'controlled traffic' flying there of third party 'unidentified' traffic at a 'reported' altitude. I think that is how it works... 019_victory.gif.9945f53ce9c13eedd961005fe1daf6d2.gif

 

 

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Guest davidh10
033_scratching_head.gif.b541836ec2811b6655a8e435f4c1b53a.gif Where is that from CFI?

Local QNH is generally sufficient and you can not expect to get an accurate area QNH (quite different to the airfield you are sitting at) given that it is a forecast average with a validity period over a big area and not particularity relevant to many localities that we tend to fly...

If you are used to reading the official weather reports, you may notice that there is often several divisions within an "Area" with different QNH. The BOM is required to indicate divisions where QNH differs by 3hp (equating at about 90'), so using local QNH is fine locally, but if you fly further afield, then you may wish to adjust your alitmeter subscale to the Area QNH for that locale. Of course you can always ask CENTRE for the QNH for a location.

 

 

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If you are used to reading the official weather reports, you may notice that there is often several divisions within an "Area" with different QNH. The BOM is required to indicate divisions where QNH differs by 3hp (equating at about 90'), so using local QNH is fine locally, but if you fly further afield, then you may wish to adjust your alitmeter subscale to the Area QNH for that locale. Of course you can always ask CENTRE for the QNH for a location.

Sure, often associated with a trough line or front. Don't get me wrong I will reset as required from any info I can get...but none of this helps nunans as he still doesn't have a sub scale.... just set local and away you go.

 

From my understanding a mode c transponder transmits your alt based on 1013.2 and is not set by the pic or am I wrong? This is from the pilot guide for the garman I have fitted to the Hornet.

ALT 

 

Selects Mode A and Mode C. In ALT mode, the transponder replies to identification and altitude interrogations as indicated by the Reply Symbol ( ). Replies to altitude interrogations include the standard pressure altitude received from an external altitude source, which is not adjusted for barometric pressure. The ALT

 

mode may be selected in aircraft not equipped with an optional altitude encoder; however, the reply signal will not include altitude information.

 

Scotty

Yeh looks like it Scotty... but it does say received from 'external alt source' and you have to wonder why it would be changing that data back to standard if it was already set to QNH... perhaps EVERYONE is sending based on standard with calc occurring at RX. No encoder no alt TX...

 

Interesting! 012_thumb_up.gif.cb3bc51429685855e5e23c55d661406e.gif

 

 

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Scotty has the principle right. Above 10.000' it's all 1013.2 and that is all the baro function of your transponder knows. ( you couldn't have a system subject to finger trouble by pilots). ATC must make an allowance for it when you are below 10.000 and flying on a local QNH,( on your altimeter)

 

The radio procedure used to be something like"ABC squawk ( discreet frequency,or 1200) and report altitude"they would get a return on radar that would be in tolerence with your reported altitude. IF the transponder is out, it can only be adjusted by a "techie" with the right calibrating equipment. Think that has to be checked every two years regardless. Nev

 

 

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My views are not the final word. I haven't used those things for around 7 years so some procedures maay have changed. I think the principle is correct. Nev

 

 

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The transponder is a radio transmitter. It transmits (responds) when it receives an interrogation signal from a radar head or an aircraft equipped with active TCAS. The basic output is mode A which is the ID code that is set on the transponder face i.e. 1200 for VFR or as assigned by ATC. If the transponder is capable of Mode C, it needs an external source for the altitude data - an encoder.

 

The encoder is a barometric device calibrated to standard ISA sea level i.e. 1013.2 and is set and checked as part of the RAD, whatever it's number is, check every two years - It is not user adjustable. The altimeter is adjustable and is checked and adjusted to correct local QNH before every flight. This can be done using either the kollsman scale with the reported QNH for the location you are at or by using the published airfield elevation set on the altitude scale.

 

If your transponder displays its altitude output you may notice that it is seldom the same as what your altimeter is reporting. This is the difference between 1013.2 and the actual QNH in the area you are in. I believe that the radar consol is aware of the QNH for each area and displays transponder returns to the operator as a corrected altitude.

 

Ok, off we launch and are tootling around and we hear centre reporting an unverified paint at 3000 feet, 10 miles east of the airfield to an inbound RPT as a possible conflict. That's where we are, so it's probably us. Being good aviators we chirp up and give a position report of our location (xx miles east of the airfield) and altitude (3000 ft) and intentions. Bingo! we're now verified!

 

* There's been a couple of replies since I started typing this.

 

 

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The encoders that are set to 1013.2 are called BLIND encoders i.e. never changes. Your transmitted alt for the transponder is referenced to the QNH. i.e. ATC allow for the QNH. For example on my transponder I can select a readout that tells me what height it is transmitting. If I check with ATC what my indicated height is the level given by ATC is the same as my alitimiter set the the broadcast QNH.

 

The correct area QNH is important for seperation - when all aircraft are on the correct QNH then seperation is achieved without reference to AMSL [especially when flying > 5000ft.] Amended QNH settings are broadcast on the AREA frequency when it changes.

 

I think someone said it before - but remember ATIS QNH is not the area QNH - the A stands for aerodrome.

 

I hope that makes sence in simple terms.

 

Frank M

 

ahlocks covered this while I was replying - duplicated response !

 

 

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T but remember ATIS QNH is not the area QNH - the A stands for aerodrome.

A is for 'A'utomatic Frank.

 

perhaps EVERYONE is sending based on standard with calc occurring at RX.

Interesting! 012_thumb_up.gif.cb3bc51429685855e5e23c55d661406e.gif

So this IS the way it works. Great discussion!062_book.gif.f66253742d25e17391c5980536af74da.gif :big_grin:

 

 

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The normal procedure is when going cross country is to set ALT to either local QNH provided by ATIS or TAF or to set ALT based on airfield height. Once at top of climb set ALT to area QNH and reset when crossing aera boundaries or aera sections as noted in NAIPS or as given by FIA. When approaching destination (start of descent) ALT is set to local QNH as received from ATIS, TAF or if neither is provided then leave on aera QNH.

 

If the ALT does not have a sub scale then the pilot will just need to take extra care when going cross country in regards to other aircraft and the stated altitude given by them.

 

 

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Tex

 

No problem here - I was just trying to point out that ATIS is not area. You obviously know and for all who know the answer ignor my post - I was just trying to indicate the diffenence between ATIS and AREA - for those who seem to have some doubt.

 

"What's in a name" the bottom line is who cares so long as everybody knows the difference.

 

I previously posted on a topic about Lowest Safe Altitude which went crasy and I regretted opening my mouth - my intent was to point out the difference between LSA and lowest planned alt in VFR.

 

If everyone has a better understanding of QHH and the associated differences then I have achived my aim in the post.

 

If it is helpful to those who are asking the questions then good - I am aware of some RAA pilots who have been flying for years but are not aware of the plus 5000 ft requirements and they only need assistance to be updated.

 

Frank M

 

 

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All good Frank... same, same.... it is the T for terminal that makes the A relevant for aerodrome as you say anyway... same result :big_grin:

 

 

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Just back from Cantberra ...

Visit parliament then?

 

I previously posted on a topic about Lowest Safe Altitude which went crasy and I regretted opening my mouth - my intent was to point out the difference between LSA and lowest planned alt in VFR......If everyone has a better understanding of QHH and the associated differences then I have achived my aim in the post.

I enjoyed that discussion about LPA, now looking forward to more on QHH.

(I'm always frivolous on a Sunday night, sorry)

 

 

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