Altimeter questions

Jaybird180

Jaybird180

Final Approach
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Jaybird180
1) How does the Mode C get it's altimeter setting? As I understand it transmits in 100's only and is not conected to my altimeter.

2) When I fly I always set the altimeter to Field Elevation and then takeoff and ATC then gives me another setting. Why should I use their setting since I just calibrated my altimeter to tell me what my true altitude is?
 
Jaybird180 said:
1) How does the Mode C get it's altimeter setting? As I understand it transmits in 100's only and is not conected to my altimeter.

2) When I fly I always set the altimeter to Field Elevation and then takeoff and ATC then gives me another setting. Why should I use their setting since I just calibrated my altimeter to tell me what my true altitude is?
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1- Mode C doesn't get an altimeter setting. It transmits the pressure altitude. (aka, the altimeter setting is fixed at 29.92) The controllers console corrects for the current altimeter setting.
2- You should always use what ATC gives you. That way your altimeter and their display should be saying the same thing. Plus it shouldn't be that far off from what you set on the ground.
 
Jaybird180 said:
2) When I fly I always set the altimeter to Field Elevation and then takeoff and ATC then gives me another setting. Why should I use their setting since I just calibrated my altimeter to tell me what my true altitude is?
Click to expand...
You need to be on the altimeter setting that the controller gives you so that everyone in the area is on the same altimeter setting which in turn greatly helps with traffic separation.
 
1) The mode C part of the transponder has it's own barometer

2) Because the other aircraft will be using that setting. Altitude separation works best when all the aircraft use the same setting.
 
COFlyBoy said:
2- You should always use what ATC gives you. That way your altimeter and their display should be saying the same thing. Plus it shouldn't be that far off from what you set on the ground.
Click to expand...

It's worth pointing out that there are two different situations possible. It may be that ATC gives you the altimeter setting for the field you just set your altimeter at, in which case you've got a measure of the error in altimeter that day. I always take note of that when I start flying. The other case is when you are getting the altimeter setting from a different setting (as an example, when I get flight following around here, the various TRACONs will most likely give me the White Plains, Islip, or Bradley altimeter settings, rather than my home field of New Haven's). In this case, the atmospheric pressure may be different at the station they're using than the field you just used, so you want to be on their setting rather than your own.
 
chucky said:
It's worth pointing out that there are two different situations possible. It may be that ATC gives you the altimeter setting for the field you just set your altimeter at, in which case you've got a measure of the error in altimeter that day. I always take note of that when I start flying. The other case is when you are getting the altimeter setting from a different setting (as an example, when I get flight following around here, the various TRACONs will most likely give me the White Plains, Islip, or Bradley altimeter settings, rather than my home field of New Haven's). In this case, the atmospheric pressure may be different at the station they're using than the field you just used, so you want to be on their setting rather than your own.
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For fields w/o AWOS/ATIS (i.e uncontrolled) I will set my altimeter to the published field elevation and then update it to whatever ATC gives me (there will be error because most airfields aren't flat). If there is AWOS/ATIS then you set it to the broadcast setting and observe the error (with a healthy amount of skepticism). Remember the recommended tolerance is + or - 75 feet from field elevation when you have an AWOS/ATIS altimeter setting. (that's a big range)
 
Jaybird180 said:
1) Why should I use their setting since I just calibrated my altimeter to tell me what my true altitude is?
Click to expand...

Helps the controller get a better idea of what indicated altitude you are at so they can relate that to other traffic they are working in your area.

I would suggest reading FAA AC00-45
True Altitude is something different all together, in fact you should actually use True alt instead of indicated alt in some situations such as low approaches near terrain and low temperatures.

ATC can give you a setting that may not show field elevation when you land...I often ask for a new setting if I suspect anything.
 
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Bob Noel said:
1) The mode C part of the transponder has it's own barometer
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I think that's a bit confusing. The Mode C part of the transponder gets its pressure altitude data from either a separate encoder which taps directly off the static pressure system (and can be located almost anywhere -- it's under the back seat in my plane), or from a combination altimeter/encoder in the panel (rare in light planes). There is no pressure measurement going on in the transponder itself. All the Mode C part of the transponder does is add the coded pressure altitude it got from the encoder to the outgoing signal.
 
COFlyBoy said:
For fields w/o AWOS/ATIS (i.e uncontrolled) I will set my altimeter to the published field elevation and then update it to whatever ATC gives me (there will be error because most airfields aren't flat). If there is AWOS/ATIS then you set it to the broadcast setting and observe the error (with a healthy amount of skepticism). Remember the recommended tolerance is + or - 75 feet from field elevation when you have an AWOS/ATIS altimeter setting. (that's a big range)
Click to expand...
Since there are airports where the elevation of various spots varies more than 100 feet from the official field el, I recommend using airport/taxi diagrams or A/FD data to determine the actual elevation where you are when you set the altimeter. This shouldn't be a big issue VFR, but it can make a significant difference if you're operating IFR.
 
The published airport elevation is the highest point on the runway. Our runway looks level but there is around thirty feet difference in elevation from one end to the other. The run up areas are still different elevations. When you set your altimeter to "runway elevation", you may not be setting it as accurately as you think.
 
chucky said:
It's worth pointing out that there are two different situations possible. It may be that ATC gives you the altimeter setting for the field you just set your altimeter at, in which case you've got a measure of the error in altimeter that day. I always take note of that when I start flying. The other case is when you are getting the altimeter setting from a different setting (as an example, when I get flight following around here, the various TRACONs will most likely give me the White Plains, Islip, or Bradley altimeter settings, rather than my home field of New Haven's). In this case, the atmospheric pressure may be different at the station they're using than the field you just used, so you want to be on their setting rather than your own.
Click to expand...
IME ATC never gives the altimeter setting for the departure airport when you contact them in the air unless that happens to be the same setting they are using for aircraft in the same sector. They will give you the destination altimeter setting if you haven't already gotten it elsewhere and you're expected to change to that when you get near the airport in question regardless of where you got it from.

ATC generally uses the same altimeter setting for all aircraft in a given sector and they want all aircraft to use that same setting when climbing to altitude or cruising. I suspect that they have some written policy that determines which station they use for the sector.
 
Ron Levy said:
Highest point on the airport operating surface, which may not be on a runway.
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From the P/CG:

AIRPORT ELEVATION− The highest point of an
airport’s usable runways measured in feet from mean​
sea level.
 
gismo said:
IME ATC never gives the altimeter setting for the departure airport when you contact them in the air unless that happens to be the same setting they are using for aircraft in the same sector. They will give you the destination altimeter setting if you haven't already gotten it elsewhere and you're expected to change to that when you get near the airport in question regardless of where you got it from.

ATC generally uses the same altimeter setting for all aircraft in a given sector and they want all aircraft to use that same setting when climbing to altitude or cruising. I suspect that they have some written policy that determines which station they use for the sector.
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JO 7110.65 tells controllers to "Issue the setting for the nearest reporting station along the aircraft's route of flight." That setting may not be readily available to the controller. IME, the setting issued is the one used by the radar data processing computer.
 
That's almost signature worthy right there. Now for the letter he once had but can't access anymore because the university kept it that says otherwise...

:rolleyes:
 
While I may have been wrong about the issue of operating surface or runway, the fact remains that there may be places on the airport that are over 100 feet lower than the official elevation, and if you want to get your altimeter set right, you need to confirm the elevation of where you are when you set it using taxiway or airport diagrams.
 
Ron Levy said:
While I may have been wrong about the issue of operating surface or runway, the fact remains that there may be places on the airport that are over 100 feet lower than the official elevation, and if you want to get your altimeter set right, you need to confirm the elevation of where you are when you set it using taxiway or airport diagrams.
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You're not sure?
 
Man, given these technical discussions that devolve into arcane spats over minutiae it is clear I do not know enough to be allowed to fly...
Funny thing though, in my ignorance I just blindly dial in the Kollsman setting the nice person at ATC gives me, fly the needles until I am 200 feet above the published field altitude, and what do you know when I look out - there is a runway and it all works out somehow... Must be magic...
 
Dr. O said:
Man, given these technical discussions that devolve into arcane spats over minutiae it is clear I do not know enough to be allowed to fly...
Funny thing though, in my ignorance I just blindly dial in the Kollsman setting the nice person at ATC gives me, fly the needles until I am 200 feet above the published field altitude, and what do you know when I look out - there is a runway and it all works out somehow... Must be magic...
Click to expand...
Or maybe just luck. Point is that if you do the altimeter check the AIM recommends before flight, and you blindly use field elevation, your altimeter could be reading 100 feet or more high without you realizing it (think about that in relation to the obstruction clearance provided at DH), or you could think your altimeter is 100 feet off when it's actually right on.
 
One of the airplanes in FBO used to have the altimeter setting noticeably off and I always set it to the altitude of the ramp. The airport diagram has the altitudes for various parts of the field, which in my case differ by about 50 ft. I was a little concerned that I'd need to fly the checkride in it and DPE would have questions. Then one day they fixed it.
 
If your altimeter is always off by the same amount, one stop-gap is to note the amount and apply a correction to all altimeter settings you get. For example, if your altimeter is always 40 feet off, and turning the Kollsman up .04 sets it to the correct elevation, then when ATC says the altimeter setting is 30.04, just dial in 30.08 and you'll have an accurate altimeter. Again, probably not a big deal for VFR, but for an ILS to mins, it can matter.
 
Ron Levy said:
If your altimeter is always off by the same amount, one stop-gap is to note the amount and apply a correction to all altimeter settings you get. For example, if your altimeter is always 40 feet off, and turning the Kollsman up .04 sets it to the correct elevation, then when ATC says the altimeter setting is 30.04, just dial in 30.08 and you'll have an accurate altimeter. Again, probably not a big deal for VFR, but for an ILS to mins, it can matter.
Click to expand...

How many altimeters are off by a constant bias?
 
Dr. O said:
Man, given these technical discussions that devolve into arcane spats over minutiae it is clear I do not know enough to be allowed to fly...
Funny thing though, in my ignorance I just blindly dial in the Kollsman setting the nice person at ATC gives me, fly the needles until I am 200 feet above the published field altitude, and what do you know when I look out - there is a runway and it all works out somehow... Must be magic...
Click to expand...

Not the point of this thread, but, there is other error in an altimeter than just a different/wrong Kollmans setting

Don't forget the "cold weather altimeter error". If you are flying in a really cold area, your true altitude will be lower, and suddenly your obstacle clearance may not be there by strictly flying that "Kollmans setting the nice person at ATC gives you".



Just my $0.02
 
Ron Levy said:
If your altimeter is always off by the same amount, one stop-gap is to note the amount and apply a correction to all altimeter settings you get. For example, if your altimeter is always 40 feet off, and turning the Kollsman up .04 sets it to the correct elevation, then when ATC says the altimeter setting is 30.04, just dial in 30.08 and you'll have an accurate altimeter. Again, probably not a big deal for VFR, but for an ILS to mins, it can matter.
Click to expand...

I value my sweet cheeks a lot more than to run any IFR approach that near mins for that much altimeter error to matter.

I like a little bit more safety factor than that.
 
ajstoner21 said:
Not the point of this thread, but, there is other error in an altimeter than just a different/wrong Kollmans setting

Don't forget the "cold weather altimeter error". If you are flying in a really cold area, your true altitude will be lower, and suddenly your obstacle clearance may not be there by strictly flying that "Kollmans setting the nice person at ATC gives you".

Just my $0.02
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It's way to easy to correct the altimeter to go flying with a error in the gauge.

Plus the fact the A&P can do it. It does not require removal from the aircraft.

simply set the Kollsman window to the Barometer setting, disconnect the Kollsman window with the little screw provided for that purpose, set the needles to field altitude, reconnect the kollsmans window, and you are good to go.
No need to open the case or tamper with the innerds of the altimeter.
 
Tom-D said:
I value my sweet cheeks a lot more than to run any IFR approach that near mins for that much altimeter error to matter.

I like a little bit more safety factor than that.
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Combine that error with other errors, like temperature and human reaction time and it could certainly be a factor.
 
Ron Levy said:
Quite a few, in my experience.
Click to expand...

Why didn't you have them corrected? Is that not a discrepancy?

Its an A&P correctable task. takes about 5 minutes.

For argument there is a difference between adjustment/calibration and a repair.
 
Ron Levy said:
If your altimeter is always off by the same amount, one stop-gap is to note the amount and apply a correction to all altimeter settings you get. For example, if your altimeter is always 40 feet off, and turning the Kollsman up .04 sets it to the correct elevation, then when ATC says the altimeter setting is 30.04, just dial in 30.08 and you'll have an accurate altimeter. Again, probably not a big deal for VFR, but for an ILS to mins, it can matter.
Click to expand...
This seems like such an obvious thing to do, that even occured to me, but it seemed too confusing. Unbelievable, I know. I thought it was safer to set to field elevation least I subtract instead of adding one day. Also my instructor suggested it.
 
Tom-D said:
I value my sweet cheeks a lot more than to run any IFR approach that near mins for that much altimeter error to matter.

I like a little bit more safety factor than that.
Click to expand...
Well, to each his own. Personally, I fly ILS's and LPV's to mins routinely, and with a 200 DH, 40 feet can be significant, especially if you're using the approach light system exception in 91.175 to continue to 100 AGL without seeing the runway itself. And I'm sure plenty of other pilots (including those not flying for a living) do the same.
 
zaitcev said:
This seems like such an obvious thing to do, that even occured to me, but it seemed too confusing. Unbelievable, I know. I thought it was safer to set to field elevation least I subtract instead of adding one day. Also my instructor suggested it.
Click to expand...
Setting to field el is fine for VFR, but you can't do that when flying an instrument approach to mins, and as noted above, it could have you setting the altimeter off by more than 100 feet at some places on some airports. Also, if you're worried about forgetting the correction amount or direction, you can do what I did on the Cheetah we used to own -- pencil a "+.04" on the plastic overlay next to the altimeter.
 
OK, same topic different direction. For those IFR folks out there; when do you change your altimeter to match the field rather then ATC assigned? For example, I frequently fly into the western NC mountains. The "last" altimeter pressure setting provided by ATC is from the big airport east of the mountains. The difference from the field pressure and the assigned can be as much as a couple hundred feet.

My assumption and what I do is change to the field pressure when "cleared for the approach".
 
Ron Levy said:
While I may have been wrong about the issue of operating surface or runway, the fact remains that there may be places on the airport that are over 100 feet lower than the official elevation, and if you want to get your altimeter set right, you need to confirm the elevation of where you are when you set it using taxiway or airport diagrams.
Click to expand...

Personally, I only set it when I'm at the runup. Each end of the runway has different elevations at my home field and the difference is 17'
 
Baron2PG said:
OK, same topic different direction. For those IFR folks out there; when do you change your altimeter to match the field rather then ATC assigned? For example, I frequently fly into the western NC mountains. The "last" altimeter pressure setting provided by ATC is from the big airport east of the mountains. The difference from the field pressure and the assigned can be as much as a couple hundred feet.

My assumption and what I do is change to the field pressure when "cleared for the approach".
Click to expand...

I do it when I get the weather at the field - which is usually about the time I'm cleared for the approach. I've never had it that far off where it's going to make a difference.
 
Ron Levy said:
Well, to each his own. Personally, I fly ILS's and LPV's to mins routinely, and with a 200 DH, 40 feet can be significant, especially if you're using the approach light system exception in 91.175 to continue to 100 AGL without seeing the runway itself. And I'm sure plenty of other pilots (including those not flying for a living) do the same.
Click to expand...

that is the syndrome the 210 pilot last year at PAE, he like to push the limits too. the first thing he saw was the approach lights in the top of his windscreen, and that was the last thing he saw in his life.

Do the google maps thing, I believe you can still see the tail of the 210 sticking out of the light tower before you get to the clif. ( ILS 16R)
 
Jaybird180 said:
1) How does the Mode C get it's altimeter setting? As I understand it transmits in 100's only and is not conected to my altimeter.
Click to expand...

They receive pressure altitude only from either an internal device (rare), a "blind encoder" box hidden behind your panel that has its own connection to your static air pressure system, or it can also be fed from an "encoding altimeter", but the setting on the altimeter has no effect on what they transmit. The blind encoder is usually the cheapest, and is stuffed up behind your panel somewhere in "traditionally" equipped aircraft. I can't speak for the G1000 or other glass panels, many probably have the data coming from the AHRS boxes, the do-all gadgets that they are...

It's actually encoded in bits, which are in a specific order called "Gillham Code" but most Avionics guys will call it Gray Code.

http://en.wikipedia.org/wiki/Gillham_Code

The number of available bits in Mode-C is low enough that yes, the resolution is in 100 foot increments. I went to look up whether the "perfect" calibration was to change over at the exact altitude, or if it happens "in-between", but I can't find that right now. Computers weren't powerful enough, and the amount of time needed to reply to a directional antenna/dedicated receiver for Mode C at a traditional "spinning antenna" type of RADAR array.

Ever note that the flat bar antenna on top of the primary RADAR concave antenna is slightly cock-eyed to the main antenna? That's because your transponder talks to that antenna, and it's slightly angled "behind" the other because it'll take your transponder a certain amount of time to respond to the pulse/interrogation from the primary RADAR.

Jaybird180 said:
2) When I fly I always set the altimeter to Field Elevation and then takeoff and ATC then gives me another setting. Why should I use their setting since I just calibrated my altimeter to tell me what my true altitude is?
Click to expand...

Yep, use what ATC gave you, their computer is correcting all the transponders to that setting on their screen. The system is set up to correct everyone to the same "real" altitude so that separation is maintained.

Here's some fun for you: If the controller gives you 29.92 and you accidentally set 29.82, how far off the real altitude he wants you on, are you, and which direction, above it or below it? (And thus, how much closer to the aircraft he just sent under/over you.)

AIM 7-2-1 is probably also worth a read... as well as FAR 43 Appendix E and F, and there's this relatively new gem...


Amendment(s) published May 28, 2010, in 75 FR 30193

Effective Date(s): August 11, 2010

6. Amend §91.217 by redesignating paragraphs (a) through (c) as paragraphs (a)(1) through (a)(3), redesignating the introductory text as paragraph (a) introductory text, and by adding paragraph (b) to read as follows:

(b) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder or with ADS–B Out equipment unless the pressure altitude reported for ADS–B Out and Mode C/S is derived from the same source for aircraft equipped with both a transponder and ADS–B Out.


Which means that if you thought you'd just plop ADS-B in your aircraft utilizing its own encoder, or a separate one, alongside your old blind-encoder or whatever... you won't... 'cause your "source" for the ADS-B Out altitude must be the same "box" as the Mode C or S transponder. Luckily most encoders in light GA aircraft have been standardized on the DE-15 connector for years, a 15-pin connector carrying 11-"bits", thus... there's some "extra" pins for ground, etc.

Check out FAR 91.217. Your encoder and your altimeter only need to be within 125 feet of each other on a 95% probability basis, and only tested with your altimeter set to 29.92 during the test.

Frankly a lot of this is "historical". The instruments and test gear to test the instruments simply wasn't very sophisticated when the system was first dreamed of and implemented. Most good shops will get you a heck of a lot closer than these minimums, depending on the age and abilities of your gear on-board. The RADAR separation standards were written with these inherent errors assumed on two aircraft, one low and one high at their certification limits, still having enough separation to miss one another in the sky.

Look up RVSM (Reduced Vertical Separation Minimums) info, for more reading on how more accurate modern equipment and especially autopilots (required to be engaged) allow for more altitudes to be used and reduced separation for busy jet altitude routes, like the Atlantic "Tracks". Only properly equipped and tested/certified aircraft can "participate".

Of course, the big iron also often have RADAR altimeters to tell them exactly how high they are AGL. A buddy who flies 777's said it was eerie one night over the Pacific to see the RADAR Alt drop to 2000' AGL. After a while, he realized that a 747 was assigned the FL two thousand feet below them, and was slowly overtaking them from behind. GPS/INS is so accurate these days, that the airliners are dead on their routes, and the 747 was a pretty big "target" for the RADAR altimeter for an hour or so until he finally puled ahead of the 777.

Fun stuff.
 
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