As ambient pressure and temperature change without readjusting the altimeter, the indicated altitude will be inaccurate. Use the mnemonics "High to Low (Pressure) or Hot to Cold (Temperature), Look Out Below" because the altimeter will indicate an altitude higher than actual; i.e., you will be flying lower than you think you are.
question, when you get into your plane, is it assumed to just zero out the altimeter to read zero to correct for temperature and pressure variances or does ATC make recommendations that are more or less than what the place shows on the ground?
When you get into your aircraft you should get the current altimeter setting from the local ATIS. Once that is dialed in you should check to make sure that the altimeter reads the value of your elevation (airport elevation). It should be within +/- 50 ' or so. Temperature variations from standard may effect this but this is the best you can do with the analog altimeter in your aircraft
Thank you for your video. I'm just curious about something. 1. Is the pressure inside the wafers always be 29.92 in Hg? If so, I think I misunderstood that the pressure inside the wafer can be changed by adjusting the knob. I'm not sure about this. 2. If the temperature is below the standard day(which is 15˚C?), does the altimeter indicate higher altitude than the average day?
넬린 Thanks for watching. Those are good questions. The wafer is sealed with gas at 29.92 Hg. This is kept constant so that the wafer expands or contracts based on the outside air pressure surrounding it. When you move the dial you are not changing anything inside the wafer, you are only moving the needle on the face of the altimeter. Think of it this way... it’s a standard day (29.92) and you are at sea level and you haven’t touched your adjustment knob so your altimeter reads 0 feet. You sit there for a few hours and the temperature goes down (and so does the static pressure) and you see your altimeter start to read more and more altitude because it is sensing a drop in pressure and thinks you are climbing. This is happening because the static pressure surrounding the wafer is now lower than the pressure inside the wafer so the wafer expands and this expansion moves the needle on the altimeter. So now it could read something like 300 feet but you know this is wrong because you are still sitting at sea level. So you decide to check for an updated ATIS weather reading and see that it says the pressure is now 29.80 Hg. So you adjust your knob to 29.80. All this does is calibrate your needle to read an altitude of 0 feet again. Nothing inside the wafer changed. So to answer the 2 questions. 1. Yes always a constant 29.92. The adjustment knob only moves the needle to calibrate you with an updated outside air pressure and 2. If the temperature goes down the pressure goes down with it and the altimeter reads this as a higher altitude. Hope this makes sense!
My instructor brought in an altimeter that we all adjusted. When the pressure knob was moved so did the other dials. Is this normal and if so how does it know my elevation based on atmospheric pressure? Thanks!
Yes. So this knob as explained in the video is the pressure setting adjustment knob. The way the altimeter works is key to understanding why when you spin this knob to change the pressure setting to your local barometric pressure that the reading on your altimeter changes. The altimeter has an aneroid wafer or balloon filled with sea level pressure air. The outside of the balloon is the air you are currently experiencing. So if the air you are currently experiencing is higher or lower than sea level it will expand or contract the ball and move the needle. However, we don’t always experience sea level pressure when at sea level. The pressure from day to day changes and is normally different from standard. This is what the what the adjustment knob is for. At sea level pressure you keep the adjustment knob at standard 29.92 as your reference that your altimeter indication base changes off of. But when pressure is not 29.92 you have to change your sea level reference point so that your altimeter now knows to compare to the new sea level pressure. For example, say the pressure is 30.12 and you are on the ground at the airport. You change your setting knob to 30.12 and your altimeter indication will change to indicate your elevation above sea level accurately because you have said hey altimeter this is our new sea level point. Then, from there as you climb in altitude and the altimeter sensing less pressure as you climb it will compare to this new reference of sea level pressure of 29.92.
I’m getting close to understanding, maybe. My local airport is at 50 feet above sea level. We adjusted to show different air pressures. That number in the window changed accordingly and the elevation dial moved also. Why? The airport didn’t climb. This is from a stationary position. Do we need to be doing addition or subtraction while flying and reading that because standard sea level elevation is 29.92 and our adjustment on the ground made the dial read X before we even took off? Thanks for taking the time to answer.
Just to add I feel like he said the altimeter he had in the class was not working because we are at sea level basically and when we set that Kollsman window to current pressure it was reading 1510 feet. Clearly not where we were sitting. Should it read zero before takeoff? Was not really happy he’s using defective instruments as examples.
@@erickfish1400 oh well if it wasn't working right Im sure it confused you more lol. But it still should have shown what happens when he changed the setting. There is a good online simulator you can play with over and over to get this down: www.luizmonteiro.com/Learning_Alt_Sim.aspx But lets say we buy a brand new altimeter and our mechanic installs it in the plane and we are first to use it at your airport. When we get in we are gonna see it set to 29.92. The altitude that it is displaying will only be accurate if our actual current pressure was a standard 29.92. So if we look at the reading and it doesnt say 50 ft then we know that the current pressure is NOT 29.92. So we would dial up ATIS or a METAR to get the latest pressure setting. Let's say we do that and ATIS says it is 29.82. In this situation our altimeter would read 150' (for now, dont worry about how I know that it might confuse you more). So we need to change our pressure setting to 29.82 so we spin the knob until it reduces in the setting window until 29.82 and as it does this, our altimeter reading will start to also drop to 050'. This is actually how pilots check their altimeter before takeoff. They make sure that with the correct pressure setting in, that the altimeter reads their known elevation (in your case 50'). ALternatively, with this knowledge we could determine the actual outside pressure without even getting an ATIS or METAR (you still want to but if you dont have access to either you can use this method). Since we know our elevation, we know what our altimeter SHOULD read. In your case it should read 50' when we are on the ground. So, we could simply spin the setting knob until our altimeter reads 50'. There is no need to do any math with your altimeter when flying or even on the ground. Simply make sure it reads your known elevation on the ground and the setting also matches what ATIS is reporting and then when flying just make sure to periodically get updates from ATC or ATIS's on the current pressure in your area and keep adjusting that altimeter setting whenever it changes for you. That way your altimeter stays accurate
Extremely helpful. I guess what I’m not getting is that the standard for atmospheric pressure at sea level is 29.92. Doesn’t that change at sea level during weather changes? That would give me a different number in the pressure window thus changing the elevation on the dial but my runway didn’t go up or down so how is that still accurate if the device is built and sealed to read zero at 29.92? Thanks again for your time, I will recommend you to all my classmates.
@@mihir777 it’s a tricky subject. An ideal gas in and adiabatic system, 100% P goes with T. In air, it’s usually also the same but gets trickier because it is not an adiabatic closed system. However, if you are training to become a pilot and preparing for your exams, remembering that Pressure goes with Temperature will help you pass the exams and understand the altimeter. In the video you sent me but deleted it somewhat misspoke. It said low pressure with a cold front when it should have said low pressure with ANY front. This makes since because air travels from high pressure to low pressure. So high pressure air will travel to meet at a low pressure area and that’s where the meeting of the two air masses occurs (front). Meteorologists usually don’t characterize a cold or warm air mass with having a specific pressure but instead say that an air mass is a mass of air with the same temperature, humidity and density. So that video was a bit misleading but I don’t blame them because it’s hard to describe such a complex system with simple rules. Here is another good video that explains this stuff ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-E-5rieCUPuc.html
@@PartTimePilot as the temperature rises, the air expands and becomes less dense. The pressure exerted by this less dense column of air above will exert less pressure. Hence, as the temperature rises, barometric pressure falls.
