View Full Version : Pressure error correction....correction
I have searched long and hard for a user friendly explanation of PEC. In the end I worked it out and felt that I had to share it with anyone else in search of the same. Hope this helps someone!!....
The RATE at which pressure decreases, decreases with height. exponentially.
The RATE at which pressure decreases, changes with a change in mean temperature of the air directly above the measuring point. That is to say if the temperature on the ground is + or - 1 deg C from ISA the RATE that it decreases in pressure from that point is changed.
Altimeters are calibrated using ISA ~ +15 deg C and they show a decrease in pressure (or increase in altitude) at a fixed rate calibrated to that temperature. At least for a while! as it becomes increasingly less accurate at displaying a true altitude as it climbs. But because we are all using the same calibration we are all experiencing the same error.
The rate at which pressure decreases with height, INCREASES with a decrease in mean temperature. :sad: That is, the pressure of a parcel of air will drop to a particular pressure at a lower true altitude than that of a parcel of warmer air. The pressure read by your altimeter on an ISA day at a TRUE altitude of 1,000ft will indicate 1,000ft but on a cold day that same amount of pressure required to indicate 1,000ft on your altimeter is actually found at a true altitude of say 900ft. So if you were to trust your altimeter you would be flying dangerously low. hence the pressure error correction.
Often a PEC table can be found in an A/C flight manual and used to determine height corrections for terrain clearance or DA adjustments. However i imagine most operators in Australia will just use a fixed buffer to save unnecessary calculations having to be made in flight, as it rarely gets that cold here in sunny Aus!
Genghis the Engineer
6th Jan 2011, 07:16
Quite wrong on a lot of counts I'm afraid.
If nobody beats me to it, I'll try and compose something to help you in a day or so.
6th Jan 2011, 08:01
Pressure Error Correction is quite straightforward.
The altimeter is "fed" with static pressure - it is, after all, an aneroid barometer.
Accurate pressure sensing depends on the position of the static source/port on the aeroplane. Various aerodynamic effects may cause the sensed pressure to be in error.
This error is termed position or pressure error.
By design, this error will be minimal when the aeroplane is in the clean configuration, but when dirty the error may be magnified.
This is the primary reason why on most jet transports the static ports and standby ports will be found on the front fuselage somewhere below the office windows.
Maybe this seemples explanation will help. :)
Im pretty sure position error and pressure error are two different things, pressure error being a temp/density related error that is of most concern in very cold conditions and is added to a DA on precision approaches...am keen to hear where i went wrong...really thought i had it there!!
7th Jan 2011, 08:55
PM shortly. :)
7th Jan 2011, 09:46
I am 100% sure Lightningmate knows what he is talking about but...
Before the both of you launch into descriptions of a technical nature check that your both talking the same lingo. I have had a similar technical chat with a downunder pilot and the sole issue was that they have slightly different names for things than we do.
7th Jan 2011, 09:48
Like shielas and dunnies mate!
Ashes is the same though......:E
7th Jan 2011, 10:19
I would defer to his knowledge of tampering with sheep though LM even if your Welsh ;-)
Genghis the Engineer
7th Jan 2011, 10:44
There are lots of different airspeeds out there, which work like this:
Indicated Airspeed - What you can see on the dial
Calibrated Airspeed - this is what you would see on the dial, if the system were perfect. It may have been nearly perfect before you wrapped an aeroplane around it, but because you did, and because there are small mechanical and pressure distribution errors within the system, CAS is not the same as IAS. The difference between CAS and IAS is PEC - Pressure Error Corrections, which are what are shown in the chart in the operators manual.
Most of the errors in the system will be in static pressure, rather than pitot pressure. So, this affects altitude as well, which explains the other graph in the manual. It's normal for the aviation boffins who create the graph to assume that all errors are static errors and work it out that way.
Equivalent Airspeed - this is CAS, modified for the fact that the ASI are designed on the assumption that air is incompressible. Below 10,000ft and 0.6 Mach, you can assume that air really is incompressible so EAS = CAS. Otherwise you can correct using a chart, or a whizz-wheel.
EAS is the most important number for structural engineers - it is basically a measure of how many air molecules pass the wing per second, and so defines all the structural loads.
True Airspeed - this is EAS, modified for changes in density. and is how fast you're actually going through the lump of space you (and a load of air) occupy. TAS = EAS divided by the square root of relative air density.
Groundspeed - this is TAS, corrected for wind. Also however, it's what you can read on a GPS, so most instrument calibration nowadays starts with GPS speeds and works backwards.
Temperature corrections to altitude are nothing to do with PEC, and are everything to do with different relationships between temperature, pressure and altitude. Your altimeter is calibrated for ISA, and will read an altitude based upon ISA conditions; if temperatures are below ISA, then the air is more dense for a given pressure, which leads the altimeter to overread - so if you were trying to clear a mountain on a below-ISA day by reference to the altimeter (a pressure instrument) you can clip the top. The standard corrections are there to avoid this. This is called Temperature Error Correction (TEC) not Pressure Error Correction (PEC).
The standard number usually taught is 4ft per 1000ft per °C. So let's say you're flying at 8,000ft on an ISA-10 day: 4 ft x 8 x 10 = 320ft, so you'll be 320ft lower than it says on your altimeter. If the mountain is 7,800ft high, you need to climb! If the mountain is 6,800ft and you're flying IFR, then you still need to climb because you're failing to maintain the legal 1,000ft separation from terrain below.
7th Jan 2011, 11:40
Just thought I'd chip in with what I've been taught (mostly on my FI(A) course);
PEC = Position Error Correction.
The altimeter does not read accurately due to the fact that local pressure fluctuations around the airframe will cause errors in its measurement. It is impossible to position a static port to reduce this error to zero.
I belive PEC for a light twin is unlikely to be greater than 50'. This is why 50' is added to minima during instrument approaches.
Obviously the radio altimeter on the autobuses means that there is no PEC to apply.
Pressure Error = Putting the wrong subscale setting into the altimeter.
Error in altimeter reading due temperature deviation from IAS = Density Error.
Standing by to be corrected by someone who actually knows what they are talking about!
7th Jan 2011, 12:34
Pressure Error = Putting the wrong subscale setting into the altimeter.
That's barometric error.
Error in altimeter reading due temperature deviation from IAS
If you meant ISA, that's temperature error.
7th Jan 2011, 12:54
Sorry, that was a typo, I did mean ISA.
Out of interest, does the P in PEC stand for Position or Pressure?
I was taught pressure.
7th Jan 2011, 13:17
Either, since they are the same thing, although correctly the error is one of pressure, due to position.
OK that was kinda stupid...good thing is its mostly a title error!! i now see that pressure/position error is simple, and as Lightning Mate described. :ok:
However it is clear that I was unknowingly trying to describe temperature error or true altitude....
Can you confirm that what i described was true in relation to true altitude?
except that altimeters do not show a decrease in pressure at a fixed rate, but at an accurate rate for ISA
And perhaps that the term "parcel of air" is a little misleading being that it is the entire column of air not just a sample of it!
and the PEC table bit!!
And 1 more question....without opening a can of worms; Does QNH = QFF at a particular location regardless of temperature, assuming both were calculated at the same place? or the same question in other words; If you are given a QNH at an A/D at an elevation of 5000ft and the temperature at that A/D is +20 deg, for the alt to read an accurate A/D elevation, wouldn't it have to have taken into account the temperature effect (non ISA) on pressure at that aerodrome?
8th Jan 2011, 10:02
Both QNH and QFF start off as the observed, actual, pressure at the station. QNH is then calculated (the met term is "reduced")down to a theoretical sea level on the asumption that ISA temps apply. The easiest way to do this is to set your altimeter to station elevation and read the subscale. This is QNH. Thus, regardless of temp or pressure at the station, with the current QNH set the reading on your altimeter at touchdown remains the correct station elevation.
QFF is different. QFF is station pressure reduced to a theoretical sea level using station elevation and the single recorded station temperature. In other words it assumes isothermal conditions from the station down.
In ISA conditions QNH should get your altimeter reading zero at msl. In cold conditions (ISA minus) your altimeter will read zero before you reach msl on the way down and vice versa. QFF, as the calculation changes to take account of station temperature, will be reasonably accurately at msl most of the time. For this reason QFF is plotted on msl forecast charts.
Note that when climbing above the station cold = danger but in desending below the station cold = safer
PS. Yes temp error calculations should take into account the temperature profile in the entire column of air, but in practice we take the ISA deviation at the indicated height and apply Genghis's rule of thumb
8th Jan 2011, 16:09
Could not have put it better. :ok:
lovely, thank you guys...that was very helpful