Altimeters

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Widgeteye
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Altimeters

Postby Widgeteye » Wed Jun 20, 2012 1:56 pm

So I'm reading along in the PHAK and I've come to the part about density altitude and am getting the impression that an altimeter doesn't really give you a true reading of the altitude you're flying but a close approximation, is that right? Or am I reading this all wrong? Because it seems to me that when you set your altimeter before you leave the airport and the temperature and barometric pressure change later on then your altimeter is no longer giving you the true altitude but the density altitude. Right? or What? :)

artp
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Postby artp » Wed Jun 20, 2012 2:34 pm

I think the density altitude, while it affects plane performance, does not affect the altimeter. The altimeter adjustment is to give you a more accurate MSL and therefore a better idea of absolute altitude, if you know the elevation of the area you are flying over.

The last thing you want on a hot day is to set the altimeter to the density altitude. That will likely result in flying into the ground while you altimeter says you are still at 1000 feet.

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drseti
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Postby drseti » Wed Jun 20, 2012 6:37 pm

Density altitude has nothing whatever to do with the altimeter reading, or the barometric pressure you set in the Kollsman window to calibrate the altimeter. It is a way of describing how aircraft performance is impacted by local conditions (temperature, humidity, and barometric pressure). It's not really an altitude at all, but rather the altitude the aircraft thinks it's at if it were flying under standard atmospheric conditions.

Let's say it's a hot, humid day, accompanied by low barometric pressure. All three of those factors make the air less dense.

Aircraft performance depends on air density three ways: with dense air, there are more air molecules flowing across the wing, increasing lift. The prop has more air molecules to bite into, producing more thrust. And, there is more oxygen going into the carbs, allowing the engine to develop more power. So, on a cold, dry winter day with high barometric pressure, the plane performs well, leaping off the ground and climbing rapidly.

Now, consider the opposite conditions. Hot, humid, and low barometric pressure all lower air density, so you get less thrust, less lift, and less power. The plane staggers into the air and climbs feebly. You say to yourself, "damn, this plane's acting as though it's at 10,000 feet of altitude!" You just mentally computed density altitude.

Of course, if you know field elevation, temperature, relative humidity, and barometric pressure, you can compute density altitude on your E6B computer. The result is the physical altitude which, on a standard day, would yield equivalent aircraft performance.

Oh, and I shouldn't have said "the altitude the aircraft thinks it's at." I really shouldn't anthropomorphize inanimate objects. They hate that! :wink:
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
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Widgeteye
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Postby Widgeteye » Wed Jun 20, 2012 7:20 pm

@drseti
I'm sorry I made you type all that, but I understand density altitude, it's the damned altimeter I don,t understand. Having never used one I'm not getting the gist of how the thing operates, if setting it to your airports pressure level in the beginning how does it keep giving proper altitude readings if the pressure levels and temperature are changing as you fly along? Say you fly into a low pressure area from a high pressure area or the temp outside climbs from 60 degrees to 98 as it easily does from morning to afternoon here in the panhandle. How does it keep giving proper altitude readings when it depends on the pressure levels you are flying through?
Thanks

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Postby FrankR » Wed Jun 20, 2012 7:59 pm

As you fly along, ATC will give you barometric pressures when you check in with them. Also you will check in with AWOS when you get to your destination untowered airport. Setting your altimeter then might make your pattern altitude more accurate. Enroute, you can check AWOS at nearby airports as you fly.
Frank
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designrs
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Postby designrs » Wed Jun 20, 2012 8:24 pm

Does baro pressure setting vary by altitude? For example, if you set your baro by AWOS... let's say it's 30.00 for a given airport which is at 000 MSL. On the same day, at the same time, above the same airport, is a baro setting of 30.00 correct at 10,000 feet?

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Postby drseti » Wed Jun 20, 2012 9:46 pm

Widgeteye wrote:I'm sorry I made you type all that, but I understand density altitude


Not to worry. That typing was not so much for your benefit, as for those lurkers out there (including my own students) who don't understand density altitude as well as you do. :wink:

Say you fly into a low pressure area from a high pressure area or the temp outside climbs from 60 degrees to 98 as it easily does from morning to afternoon here in the panhandle. How does it keep giving proper altitude readings when it depends on the pressure levels you are flying through?


First off, although air temperature has a huge influence on density altitude, its impact on an altimeter's reading is entirely negligible. So, the only thing you have to worry about is changes in atmospheric pressure. As others have mentioned, there are a number of sources of corrected or updated barometric pressure readings available in flight, and you can (and should) update your altimeter setting from time to time (especially just before descending into the traffic pattern at your destination airport). This is one of the reasons why we have various local weather transmissions (ATIS, ASOS, AWOS, DigiWx, etc.) available at airports. By all means, reset your altimeter to local field conditions before landing!

That said, this is a problem that's far more critical for IFR operations (where you can't see the ground) than it is for VFR flying. With experience, you will be able to judge traffic pattern altitude pretty accurately, without ever looking at the altimeter. Also, during the course of a flight, the changes in barometric pressure are likely to be small, meaning altimeter error will not be great. A 0.1 inch discrepancy in altimeter setting translates to a 100 foot error -- not particularly critical for VFR flight (but potentially lethal when you're shooting an instrument approach down to minimums).

With regard to terrain clearance, your example (flying from a low into a higher pressure area) is not so important, as the resulting altimeter error will have you ending up higher than you thought. What's critical is flying from a high toward a low. Now the altimeter error, if not corrected, will be in the opposite direction, and you'll be closer to the terrain than you thought you were. IFR pilots remember this from the mnemonic "from high to low, look out below."
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
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Postby drseti » Wed Jun 20, 2012 9:56 pm

designrs wrote:Does baro pressure setting vary by altitude?


That's a very astute question, designrs! Of course, atmospheric pressure varies predictably with altitude -- that fact is what makes altimeters work! At low altitudes (i.e., below 10,000 feet, where Sport Pilots fly), the relationship is relatively linear: pressure decreases about 1" of mercury for every thousand feet of altitude increase.

However, the barometric pressure reported by airports, and to which you set your altimeter, is not really absolute atmospheric pressure at that location. It has been corrected to sea level. If it weren't, when you dialed in the pressure at any airport (at any physical altitude), your altimeter would read zero feet! (Since you want to know your altitude relative to Mean Sea Level, your reported altimeter settings have to have been corrected to sea level.)

Consider this example: on a standard-atmosphere day, barometric pressure at sea level is 29.92 inches of mercury. Sitting on the ground at an airport 1000 feet above sea level, the column in a mercury barometer would actually rise 28.92 inches. The reported altimeter setting would be 29.92 and, sitting on the ground and using that setting, your altimeter would show you 1000 feet above sea level. If they reported the actual mercury barometer setting, and you set that in the Kollsman window, your altimeter would show you at zero feet (which is your altitude AGL, not MSL -- and not particularly useful when you fly away from that airport).
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
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jnmeade
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Postby jnmeade » Wed Jun 20, 2012 10:24 pm

An altimeter is a barometer. A barometer reads 29.92 in. mg at 59°F at sea level on a day with standard pressure. The pressure is the weight of the column of air. Let's suppose you made the air more dense by making it cold or by piling up more air. It would weight more. Maybe it would displace 30.42 in mg. The lapse rate of pressure is about 1 in mg per 1000 feet at lower altitudes. Does this high pressure area move the ground down 500'? No, of course now. But if you have a standard barometer you will thyink you are 500' below sea level To get your altimeter to show you are still at sea level, there is an adjustment mechanism. We could set the altimeter accurately by moving the knob until the needles pointed to 0'. Or, we could move the know until the Kollsman window showed 30.42. Both do the exact same thing.

If we always flew in day VFR, maybe we wouldn't need a "sensitive" altimeter as this is called. You probably would want one to be sure you were above obstacles and at the right altitude to enter the pattern to land.

But, we may fly at night and we may be mingled with traffic flying on IFR. Now, we have to worry about separation from terrain and airplanes that we may not be able to see.

We know our altitude above msl by setting our sensitive altimter to the local barometric pressure. It gives us an indicated altitude. We change our Kollsman setting by tuning in the local ATC or weather facility enroute and using the closest one.

By keeping a local barometric pressure on the altimeter, everyone uses the same reference. This is important for traffic separation. Here's an example.

You are flying east at 7,500 feet and the local barometric pressure setting is i29.92. Over a long trip, the pressure may change. Let's say it's now 30.42. That is a .5 in mg change, or 500'. (Pretty big change, but I've seen it.) If you were sitting on the ground and this happened, the altimter would all of a sudden show you were at -500 feet. What would you do if you were flying and saw this on the altimeter? You'd pull back on the stick until it showed 0'. Same at 7500'. As the pressure increases, the altimeter responds by showing lower numbers, as if it were beneath a higher column of air. You respond by pulling back on the stick to get the number back to 7500. But, since you didn't change your Kollman number, you are really at 8,000' when the altimeter shows 7500.

8000 is the westbound IFR altitude. Plenty of pilots file IFR in clear daylight and one may be heading straight toward you.

Let's say there are no IFR pilots but way to the east another VFR pilot took off and he is going to use the westbound altitude of 8500'. The pressure is really high there and when he took off, he set his Kollsman window to 30.92, the local barometric pressure. As he flew west, forgetting to update his altimter setting, the altimter sense lower and lower pressure. As it gets to your new location which is at 30.42, his altimter would like to show 9000, but he has been compensating all along as the altimter climps up he has been desceding. His altimter shows 8500 but he is at 8000 feet, the same as you.

You may say these are exaggerated numbers and I agree, but what is more likely is that you take off and fly east at 7500 but actually creep up to 7700 and he flies wesat at 8500 but actually lets it sink down to 8300 indicated. Now all it takes is a .6 inch change and you two are on the same level.

Up to FL180 you are working with indicated altitude. Above FL180 (18000) you fly on pressure altitude - everyone sets the altimeter to 29.92 until you go below that altitude. You do not reset your altimeter.

BTW, your transponder always sends pressure altitude and never adjusts itself for nonstandard pressure. ATC figures the corrections to adjust your pressure altitude to indicated altitude. You don't worrry about this as you can't do anything about it.

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designrs
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Postby designrs » Thu Jun 21, 2012 1:02 am

Wow! Thanks to all for such thorough answers!
Taking the question one step further, if I may, does the baro setting effect the airspeed indicator?

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Postby jnmeade » Thu Jun 21, 2012 8:05 am

widgeteye, to get back to your OP - everyone adjusts their altimeter to local barometric pressure so everyone is reading off the same sheet of music in regard to altitude. If you're off, you're all off by the same amount in the same direction. There is no way to adjust the altimeter for different temperature or humidity or other things that combine to create density altitude so everyone ignores it (The except is that at some atmospheric conditions ATC may restrict flight just at or above FL180 if it might conceivably let traffic on pressure altitude get too close to traffic on indicated altitude. This does not happen very often, but sometimes a plane will requirest FL180 and be required to use FL190.)

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Postby Widgeteye » Thu Jun 21, 2012 9:21 am

Let me make sure I'm clear here. When you say corrected to sea level, do you mean the standard sea level pressure corrected to changes in barometric pressures from 29.92? like say yesterday the barometric pressure here was 26.22 this morning it is 26.47 there has been a .25 increase in pressure during the night due to a high pressure system moving into the area. When the correction is made are they correcting the standard pressure of 29.92 by the amount the pressure has changed? For instance say yesterday they would have reported 29.92, would they today, report it corrected to 30.17?

Sorry to complicate this but I have to get it straight in my head. :)
Thanks

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Postby drseti » Thu Jun 21, 2012 9:30 am

Let me 'splain it, Widget.

Let's say the reported altimeter setting is 30.00 inches of mercury. If your airport is at sea level, and you set up an old fashioned mercury barometer, the mercury will rise in the glass tube exactly 30.00 inches. If your airport is at 1000 feet above sea level, that same mercury barometer will show about 29 inches in the column of mercury (about one inch less than it did at sea level, because the air is less dense at 1000 MSL). If your airport is 2000 feet MSL, the column of mercury will go up about 28 inches. In all three cases, the altimeter setting will be reported as 30.00, so the altimeter will indicate 0 feet MSL, 1000 feet MSL, and 2000 feet MSL, respectively. That's what we mean by saying the altimeter setting has been "corrected to sea level pressure".
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
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Postby drseti » Thu Jun 21, 2012 9:42 am

designrs wrote:does the baro setting effect the airspeed indicator?


Yes, it does. Consider how an airspeed indicator works. It essentially counts the number of air molecules hitting the front of the pitot tube. If the air is less dense, there will be fewer molecules, hence the needle will not go up as high. Thus, there is a difference between "indicated airspeed" and "true airspeed", which varies with altitude, temperature, humidity, and barometric pressure.

The way you compute true airspeed is to read the airspeed indicator, set your altimeter to 29.92, read pressure altitude off it, look at the outside air temperature gauge, and then enter all three of those numbers into your E6B. Out pops TAS. (Note that we generally don't correct for relative humidity, because its impact is relatively small. But water vapor is less dense than nitrogen, hence steam rises -- so, technically, we could factor in humidity as well, if we wanted to be really precise.)

Of the various factors that influence air density, altitude is generally the dominant one. So, we can approximate TAS from IAS and altitude alone. To a first order, at the lower altitudes IAS drops about 2% per thousand feet of altitude increase. So, if you're flying at 5,000 feet, and your ASI indicates 100 kts, your TAS is about 110% of 100 kts, or 110 kts. This will get you pretty close, most of the time.

One further note is that the aircraft performance depends upon airflow, and so does the airspeed indicator. So, all your performance numbers (Vso, Vs1, Vx, Vy, best glide speed, landing speed, rotation speed, etc.) are all based upon indicated airspeeds. Just fly everything to the same numbers on the ASI, regardless of conditions, and the plane will respond predictably. The only reason to care about TAS is in computing time enroute and fuel burn (and when bragging about your aircraft's performance).
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

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Postby drseti » Thu Jun 21, 2012 9:44 am

jnmeade wrote:An altimeter is a barometer...


That was one of the most clear and succinct explanations I've ever seen. Thanks for posting it!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying


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