Altitude

[skeptic36]

Hi all,

 

I guess this is something I should have asked my instructor, but it has never occurred to me as all my flying so far has been around similar elevations.

 

Prior to taking off I set the altimeter to zero and away we go. When advising my position in radio transmissions I read my altitude from my altimeter and that is what I say.

 

So what should I do if ground level changes to a significant degree ?

 

Should I do a mental calculation then advise my position amsl ? I would think that would complicate things for local traffic, but I can't see how else I could give an accurate description of my position :confused:

 

Regards Bill

 

 

[drifter_driver]

the altimeter should be set at area QNH or dial the elevation of the field you are flying from into the altimeter as part of pre take off checks. any other settings will bring you in conflict with other traffic using correct settings

 

 

[Guest drizzt1978]

As the aerodrome I fly out off, is 10 feet above sea level, so thats what we set it at (aproximately)

 

So when you fly in that area thats what everyone else is working on.

 

But when you fly into a new area you set that Qnh which gives you that rough area height, so technically if everyone is doing the same thing there should be no confusion.

 

Make sense?

 

Hope so!!;)

 

 

[Skyhog]

Get a new instructor!!!

 

 

[turboplanner]

There's quite a good thread on QNH and its implications for traffic somewhere on his site. It would answer your query, and more.

 

 

[Guest Mad Dave]

Back to the original question there is no need to do a mental calculation if the ground level changes, as your altitude is AMSL, your height is above ground level, in your radio transmissions you are reporting your altitude, height is only important for clearance. If you are going some distance, keep your ear on the centre frequency, and they will be giving the QNH to other aircraft, you can then set it on your subscale to make sure you are right.

 

Dave

 

 

[ossie]

Just out if interest Skeptic36 what's the airfield elevation where you fly from ?

 

 

[skeptic36]

Hi all,

 

Thanks for the replies so far.:thumb_up:

 

Ossie, Great Lakes is 269' amsl

 

Dave, are you saying I should be using agl when around airports and amsl when not ?

 

I thought amsl would only be used above 10,000'

 

Turboplanner, the only thing I can find was Maj Millard not being able to get QNH.

 

Skydog, I don't think it is the instructor at fault, I did my tuition under HGFA and I have just been through the work book cover to cover and the only reference to QNH is in the definitions section ie. define the following aviation terms and abreviations: QNH: air pressure value that represents sea level at a particular place and time. So I think maybe it is something HGFA should be fixing in their course, maybe they think we don't fly far enough to be concerned.

 

Drizzt1978, what if I'm flying between Great Lakes and Mt Hotham, how do I give an accurate position when I'm 5nm NW of Swifts Creek unless I use amsl ? i_dunno

 

Thanks again

 

Regards Bill

 

 

[Guest Mad Dave]

G'day skeptic36,

 

What I was trying to say is when you set your altimeter to your airfield elevation (basically setting the QNH), you are then using your height AMSL for your altitude, your height (AGL) will also depend on the elevation of the terrain you are flying over.

 

So if your home elevation was 100' and you were doing circuits at 1100' (1000' AGL), then you flew to a nearby airfield where the elevation was 200', you would join the circuit at 1200', but you would still be 1000' AGL.

 

Does that make sense?

 

 

[Guest Cloudsuck]

G'day skeptic36,

 

Before departing the strip you shoud set your altimeter to 269 ft (field elevation). Your circuit height at that strip shoud be 1269 ft (which is 1000 ft agl).

 

When you arrive at your destination field, you look up ERSA and see that the field is 450 ft AMSL and you do your cct at 1450 ft indicated (which is 1000 ft AGL). If you don't do this and you do it your way, you arrive at the destination you will do your cct 181 feet lower (AGL) than everyone else (450-296 =181).

 

Setting either field elevation or QNH before you depart will give roughly the same thing. As said above, I listen to the area freq in flight and update the QNH as I go.

 

To give another example, when I was a HGFA pilot (Paraglider), I would launch off a 1800 ft mountain. I would set my altimeter to 1800 ft before launch because the mountain is 1800 ft AMSL. So when I thermal up to 7500 ft, that is reading 7500 AMSL (same as every other aircraft in the sky). If I had zero'ed the altimeter on the mountain I would be reading 1800 feet lower than everone else (eg. telling everone I'm at 5700 when I'm really at 7500).

 

Zero'ing the alimeter on the field really has no place in aviation except if you are doing ccts at that field.

 

 

[Guest Mad Dave]

Yeah,

 

I was just thinking about what Cloudsuck says, if you flew to an airfield with an elevation of 1100' you would be joining the circuit underground!!

 

 

[Guest brentc]

Bill,

 

You need to quote ALL altitudes in AMSL format. There is no operational requirement to quote your altitude in AGL with the exception of perhaps informal discussion when chatting with other pilots. Eg, I'm at Hotham doing a 500ft circuit or letting someone know your approximate position with regard to a local reference.

 

So as stated by previous posters, at Lakes Entrance you will set your altimiter to 269' before takeoff (assuming you are at the top end of your runway there!)

 

When you come back to Lakes Entrance, you will quote your altitude as it reads on your altimiter and you would to your circuit at 1,269' or thereabouts if you are doing a 1,000ft circuit.

 

Agreed that complexity does take hold when you are on a cross country if you are hoping to fly at 500ft agl in the middle of nowhere as you won't know the height of the ground and can only estimate your 500ft. Then along comes the Garmin 296 which can tell you if you are 500ft from the ground at all times

 

 

[turboplanner]

The QNH thread was quote a long one, but the older threads are tedious to find because they sometimes start out as a different subject. A word and phrase search capacity would be good, depending on the cost and Ian's time.

 

AGL is critical in the circuit where you have to maintain separation from other traffic. If your circuit height is 1000 feet AGL you need to be there, not 1050 feet.

 

In your case, Great lakes, if there is no external QNH reference you would set 269 at the location where that had been measured (Great Lakes shows 260 in ERSA).

 

At that point, which is GL, AMSL will be the same AT THAT TIME, and you can reverese read AREA QNH for Great Lakes.

 

You could then radio that QNH to an incoming pilot.

 

When you take off, your altimeter, reading AMSL will read 1269 when you are downwind at 1000 feet AGL.

 

The incoming aircraft, having read the ERSA will also be downwind at 1269 feet on his altimeter.

 

If you just zero the altimeter, and turn downwind at 1000, you could be in conflict with an incoming AC flying on QNH (Unless your airfield is at sea level), but it breeds bad habits.

 

I was caught on an early navex flying in to Albury, and wondering why I could see the leaves on the bushes etc - Albury elevation is 539 feet, so I'd joined downwind at about 500 feet. Luckily no one else was around.

 

Accurate AMSL is important in the cruise for separation and also in marginal conditions to be able to read off the altitude of ground obstacles, and know you'll have enough clearance.

 

While your obligation for RAA licensing is RAA procedures, the PPL Performance and Operations theory segment is fascinating and will save you a lot of heartache in planning.

 

It goes into QNH, QFE, and QNE and how to calculate pressure height.

 

It also shows you how to plan the entire trip from a performance aspect.

 

It explains how to input your passenger and fuel loads, and the temperature, and calculate how much runway you will need. You'll find there's a staggering difference as the day gets hot, and many pilots have gone though the end fence and been killed, just for want of this simple calculation.

 

It gets into fine detail to calculate fuel required by calculating fuel burn in the climb, converting Indicated airspeed into true airspeed etc.

 

Clear Prop may be able to get you a CASA based ppl Performance and Operations book, but failing that, mail order from the flight shops.

 

 

[Guest pelorus32]

Skeptic36,

 

you've had great advice on this thread. Just for completeness remember that when you set the airfiled elevation on your altimeter prior to take off you can read the QNH in the Kollsman window - as others have said. This is the "actual" QNH. It's quite possible for someone else to arrive at your airstrip with the official Area QNH set and for this to differ from your "actual" QNH by a few hectopascals. Unless this difference is great the effect isn't that much - about 30 feet per hectopascal (at ISA it's actually 27.3 feet).

 

Regards

 

Mike

 

 

[GraemeK]

I usually try to cross-check things by setting the altimeter on the airfield elevation, then having a quick look at the Kollsman window to see if the setting is close to the local QNH (for YLIL I use the YCEM QNH off the 'net).

 

Just catches gross errors ......

 

 

[Yenn]

Bill What you are using is QFE. That is only suitable for doing circuits. What you need is QNH, which has been explained above. But to re iterate just set your field height on the altimeter and you will be near correct.

 

 

[Guest brentc]

If you just zero the altimeter, and turn downwind at 1000, you could be in conflict with an incoming AC flying on QNH (Unless your airfield is at sea level), but it breeds bad habits.

Well not quite

 

If you are at Sunbury where it's say 1,100 feet or thereabouts ground level, you set your altimiter to 0 feet on the ground, then fly the circuit at 1,000 ft, you are still flying a 1,000 ft circuit :big_grin:

 

The only conflict comes about with the use of the radio which is what Bill was originally asking about.

 

Where you will come un stuck is flying into Hotham at over 4,000ft, you can't turn your altimiter QNH far enough to get to Zero feet!

 

We also need to remember that there are many aircraft out there in RA-Aus that don't have altimiters with QNH subscale's fitted, such as aerochute's and some minimal trikes, so you've still got to look out those windows in and out of the circuit!

 

 

[turboplanner]

Yes, I could have chosen my words better. You will be at 1000' AGL but the conflict comes when the other guy comes in from a long way off on the last QNH he picked up. If he asks you for QNH, which you got from a ground reference of 0, how are you going to give it to him?

 

 

[Guest brentc]

If you are at Mt. Hotham, you can't (because your qnh will be off the dial).

 

That's why procedures are in place for obtaining QNH from Centre on flights over 100 miles etc. I wouldn't trust a QNH that someone gave me if I had to rely on it.

 

Reminds me of something interesting.

 

We have a QNH on our airfield website complete with disclaimer that it's not actually at the field, but nearby. Recently a nearby charter operator (could even be rpt) called me up because the QNH wasn't working. Turns out they were using the QNH as a valid QNH for their IFR approaches! (mind you, sometimes in some VERY nasty weather!) Needless to say that when I found out about it I quickly changed the disclaimer again to recommend against this type of activity.

 

 

[skeptic36]

Hi all,

 

I am sure I have it figured now.

 

I have been thinking about this all day, mostly how I didn't get this worked out during my training. It has become clear there was a deficiency in my training when I remember being instructed to zero the altimeter as part of the pre take-off checks and no other work was ever done with it. I can see why the instructor would use that method with new pilots flying locally around the airfield but he certainly should have explained the correct method prior to course completion. I had no idea how to read qnh off my own altimeter (thanks Mike) I thought that was something you got from air traffic controllers or weather forecasts, lucky I haven't been asked for it yet.

 

It seems strange there is nothing about this in the HGFA theory work book, I'd be interested to hear from other HGFA trained pilots about what they where taught on this subject.

 

Thanks to all contributors

 

Regards Bill

 

 

[Tracktop]

Hi all,It seems strange there is nothing about this in the HGFA theory work book, I'd be interested to hear from other HGFA trained pilots about what they where taught on this subject.

 

Regards Bill

Hi Bill

 

I am training under HGFA at present. Trained to set altimeter to QNH ( airfield height above MSL) before takeoff.

 

My guess is that as you are asked the definition of QNH and the require reference is "Microlight Pilots Handbook by B Cosgrove" I think the expectation is that you would read and understand the text. I must admit I did look elsewhere as well for other descriptions as well in an effort to understand the concept. I guess ensuring you have the correct understanding is ultimately the instructors responsibility.

 

I have so far trained with 3 different instructors from 2 different schools and they all ensured I set QNH correctly before takeoff.

 

Ray