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Altimetry

Asked by: 2113 views
Aircraft Systems, Commercial Pilot, Weather

Hello,

 

  • Say you are flying at a true altitude of 5,000ft MSL with an altimeter setting of 30.00. You fly into an area of low pressure where the altimeter setting should be changed to 29.00, except you don’t change it, it still shows 30.00. Will you be flying above or below the altitude shown on your altimeter? Explain why.

​Below. At first, when the altimeter setting is accurate (as in, when we are flying in an atmosphere where the sea level pressure is actually 30.00), the pressure outside the plane at 5,000ft MSL is approximately 25.00 (30.00 - 5 inches, i.e. -1 inch for every 1,000ft of altitude gain). At this point, the accurately-set altimeter senses 25.00 and indicates our true altitude: 5,000ft. From here, as long as the altimeter setting doesn’t change, the altitude will always read 5,000ft when the altimeter senses a pressure of 25.00. Once the low pressure system moves in, the new atmosphere has a sea level pressure (the altimeter setting) of 29.00, meaning that the ambient air pressure of 25.00 that our altimeter is sensing (and we are tracking, because it corresponds to 5,000ft) is now found at a true altitude of 4,000ft (29.00 - 4 inches, i.e. -1 inch for each 1,000ft of altitude). Because our altimeter is essentially just a barometer that tracks pressure levels, it will track the 25.00 pressure down to a true altitude of 4,000ft. The altimeter is still using the old pressure scale where sea level pressure is 30.00, though, and on this scale the ambient air pressure of 25.00 will always correspond to 5,000ft. So 5,000ft MSL is still showing on the face of the instrument, and we are flying a true altitude 1,000ft below the altitude we think.

I was always taught that when we enter a low pressure system, the altimeter senses a climb due to the decreased air pressure, allowing the aneroid wafers to expand, and therefore we are flying below indicated altitude.

However, the decrease in pressure actually causes the aircraft to descend 1000' due to the decreased density of the air. Since we still have 30.00 set on the altimeter, it will still indicate 5000'. when the altimeter is changed to its correct setting of 29.00, the altimeter will still sense a pressure of 25.00 but now indicate 4000.

is this a common misconception among other student pilots? 

Also, do changes in temperature affect the aircraft in the same manner? Meaning, does an increase in temperature cause the craft to climb due to the increased presence of air molecules at altitude, or does it only cause an decrease in indicated altitude due to the higher presence of air molecules, or both? and the opposite for cold air, which would cause an increase in indicated altitude, and/or a loss of true altitude?

 

 

 

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1 Answers



  1. John D Collins on Jan 31, 2021

    Pressure changes in flight are so small as to be unnoticeable with slow changes in the altitude indication. It is the pilot or autopilot that adjusts the vertical flight path if the indication changes. For an autopilot with altitude hold, they either sense a reference pressure and use the pitch or pitch trim servos to make an adjustment to hold the same pressure or they are able to determine a digital indication of the altitude and make an adjustment to maintain the selected altitude indication. Pilots will normally climb or descend to keep the desired altitude indication the same. It is the Kollsman window pressure setting that adjusts the reference for the altimeter indication to reflect the standard pressure lapse rate into a corresponding MSL indication. The standard pressure lapse rate does not take into account non standard lapse rates or non standard ISA temperatures, so the MSL indication can have significant errors verses the true altitude, particularly with temperatures different than ISA.

    At the station location where the altimeter setting is determined, temperature and pressure are accounted for. In the old days, at my FBO, we used to have two certified altimeters on a vibrate box. We would set both altimeters to the known elevation of the altimeters and turn on the vibrator, adjust as needed. The average of the two altimeters Kollsman window setting was the barometric altimeter setting we would radio to pilots. Now an ASOS automates the determination. Since the altimeter setting is based on a known altitude at a known location, any pressure or temperature anomalies are taken into account at that point. But as you move higher in the atmosphere or further from the site used to determine the altimeter setting, one depends on the standard lapse rates to determine the altimeter indication. That means that the higher you go, the more error is introduced by non standard temperatures. A warmer temperature will cause the altimeter to read below its true altitude and a colder temperature will cause the altimeter to read above the true altitude. The natural gas law says PV = nRT or to put it another way, the pressure increases with higher temperature. So a lower temperature will cause a lower pressure and therefore indicate a higher altitude on a barometric altimeter. Conversely, a higher temperature will cause a higher pressure and therefor indicate as a lower altitude on a barometric altimeter.

    As a side note, GPS altitude is unaffected by temperature and will indicate a truer actual altitude, usually within +/- 30 feet. We don’t use the higher accuracy for separating traffic vertically because not every aircraft is equipped with a GPS base altimeter. Aircraft below the Flight Levels are supposed to adjust their altimeters to a station within 100 NM and if you fly in the Flight Levels, every aircraft sets the altimeter to 29.92. In fact, we don’t rely on MSL altitude either for radar, ADS-B, TCAS, TAS systems, transponders, or traffic in the cockpit. We only use pressure altitude where the altitude is always referenced to 29.92, so there is no dependency on the pilot knowing or entering the correct altimeter setting. Also, ever wonder why in the mountains we are required to maintain 2000 feet above the terrain with our MSL altimeters? It has much to do with temperature and sparsity of locations where altimeter settings are determined. If you want to avoid hitting the rocks, the GPS altitude is much much better than your baro altimeter.

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