from high to low look out below

[justin31]

Hi there I seem to be having some trouble grasping this phrase which is see in almost all books and I am hoping that some one here can help me with it.

So first of all it is a fair statement to say colder temperature means increased pressure? it is also fair to say that the higher you get the lower the pressure gets because of the weight of the column of air above?

So if you go from a high pressure area to a lower pressure area does that not mean the altimeter will indicate an increase in height? or if it is talking about temperature, the cold temperature would once again mean decreased temperature which the altimeter will indicate an increase in your height.

So does this phrase basically tell you that "hey if you run into a low pressure area don't push the yoke forward and go back to your original altitude because its a dirty trick!"?

I don't know why I have trouble grasping this but hopefully someone can tell me if I am write in what I have said.

Thanks!

Oh 1 more thing, is it kind of stupid for me to ask the instructor to help me judge a mile when i am up with him? Every time I have gone up with any instructor and even my first flights it seemed to me like it was something I am supposed to know and the instructor just expects you to know.

Thanks.

[Shiny Side Up]

The best way to understand the "high to low" problem is to realise that the altimeter doesn't magically measure how much weight of the atmosphere is below you but rather it measures the weight of the atmospere above you. Think on that first, and I'll see if I can find a diagram to link, though I know there's a good diagram in the FTGU and another in the Air Command book which explain it.

As to the second part, if your instructor assumes something that you have trouble understanding or grasping - ask. That's what he's there for. He should be able to give you some examples to help you out in regards to the judging distance question. I know around here its easy to demonstrate as the world is neatly divided up into squares a statute mile per side, but you may be in a less convinient place.

[BTD]

So if you go from a high pressure area to a lower pressure area does that not mean the altimeter will indicate an increase in height?

You are correct. The primary meaning of the statement regards the hazards of not adjusting your altimeter setting when flying in areas of differing pressure.

Remember your altimeter does not read altitude above sea level it reads a certain pressure level calibrated into feet and only by setting the correct altimeter setting do you get a correct above sea level altitude. If you don't reset your altimeter when flying from high pressure to low pressure that particular pressure level is getting closer and closer to the ground and in an extreme case you could end up flying into the ground.

If you take your example above, the altimeter reads higher therefore you are going to correct by descending. It happens in small changes though so you never realize.

Colder air generally means higher pressure on the ground, but in flight because it is more dense the pressure levels tend to be closer together, than they are in warmer air. Down the line look up Thermal wind component.

Also, if you are having having difficulty judging distance by all means ask your instructor. Another method is to picture the runway you are familiar with and lay an imaginary number of them end to end in your head to help you judge. If your field is 6000 ft long theres a mile. If it is 3000, thats half a mile. etc etc.

Hope this helps.

BTD

[justin31]

OK thanks for the replies guys and I will check again in FTGU and the weather manual and I will check into the thermal wind component.

So just to confirm once more--the saying is for pilots who end up into lower pressure and see that the altimeter indicates a climb, so what they will do is descend to maintain there previous altitude but in actuality they did not descend nor climb but just the altimeter indicated that error.

[North Shore]

^Not quite. As you progress from high pressure to low pressure, your altimeter will register a climb. You, being an observant and precise pilot, will notice this, and correct to stay at your pre-selected altitude by descending. However the actual distance between you and the ground will have decreased - thus the "look out below" bit. Not really critical in VMC in the Prairies, but if you are in the rocks somewhere, and plan to cross a pass with 500' (which has now shrunk to 200') to spare, then it could become critical quite quickly.

[BTD]

This is the best picture I could find to show the issues.




Notice the entire time the aircraft altimeter shows 5000ft. But the 5000 ft pressure level is getting closer to the ground. So the pilot maintains 5000 ft indicated, but he is getting closer to the ground because that pressure levels is.

[C-GPFG]

If you maintain a constant indicated altitude as you fly into an area of lower pressure without adjusting your altimeter your TRUE altitude will decrease. Conversely, I use to teach "From low to high you're going up in the sky"..which could be an issue for my cross-country students returning to the airport which lies under the YYZ terminal control area.

[justin31]

Okay once again thanks for the replies and also for the illustration. You are all saying that your height above sea level will be lower when you enter the low pressure area compared to when you were in the high pressure area, but doesn't low pressure mean less dense air which you will find high up in the atmosphere or air with less weight above it? So how would that indicate to you that you are lower then you are supposed to be (look out below)? or in terms of temperature the colder temperature means more high pressure so I end up back at my question. Or do you guys mean low as in lower temperature which is dense and sinks to the earth and something towards this line?

Thankyou

[North Shore]

Justin, you're making this way more complicated than it is!

The altimeter is simply a barometer (measures air pressure) that reads off in feet. If we build a giant measuring stick 2500' up into the sky, and fly beside the top of it, then we'll know that we are at 2500 feet, and set our altimeter accordingly. The 'weight' of air above us pressing down on our altimeter gives us a pressure level of 2500'. As we fly away from that 2500' measuring stick, and the atmospheric pressure changes, then the altimeter will read off the change in pressure as a change in altitude. So, as we fly into a low pressure area, the 2500' pressure level that we set earlier next to our measuring stick is actually closer to the ground than our 2500' stick - we have to fly lower in the atmosphere to maintain the same 'weight' of air above us to read 2500' on our altimeter. Conversely, if we fly into a high pressure area, then we can fly higher to get the same 'weight' above us...

Before I can get to the bit about temperature, I'm going to have to read FTGU to get it straight in my mind - so I'll defer to someone else..

[justin31]

Justin, you're making this way more complicated than it is!

The altimeter is simply a barometer (measures air pressure) that reads off in feet. If we build a giant measuring stick 2500' up into the sky, and fly beside the top of it, then we'll know that we are at 2500 feet, and set our altimeter accordingly. The 'weight' of air above us pressing down on our altimeter gives us a pressure level of 2500'. As we fly away from that 2500' measuring stick, and the atmospheric pressure changes, then the altimeter will read off the change in pressure as a change in altitude. So, as we fly into a low pressure area, the 2500' pressure level that we set earlier next to our measuring stick is actually closer to the ground than our 2500' stick - we have to fly lower in the atmosphere to maintain the same 'weight' of air above us to read 2500' on our altimeter. Conversely, if we fly into a high pressure area, then we can fly higher to get the same 'weight' above us...

Before I can get to the bit about temperature, I'm going to have to read FTGU to get it straight in my mind - so I'll defer to someone else..

yeah lol I know I am making this to complicated but not sure why!

So okay the stick is at 2500 and we are flying beside it and yes altimeter will say 2500 ASL, now if we go into a low pressure area which means less density which means a increase in the altimeter reading, the pressure level which we used to maintain the 2500 beside the stick is still 2500 ft ASL but now we are in decreased pressure meaning the stick is below us or yea closer to the ground.

right?

[BTD]

Okay once again thanks for the replies and also for the illustration. You are all saying that your height above sea level will be lower when you enter the low pressure area compared to when you were in the high pressure area, but doesn't low pressure mean less dense air which you will find high up in the atmosphere or air with less weight above it? So how would that indicate to you that you are lower then you are supposed to be (look out below)? or in terms of temperature the colder temperature means more high pressure so I end up back at my question. Or do you guys mean low as in lower temperature which is dense and sinks to the earth and something towards this line?

Thankyou

Once you see it, you will be kicking yourself. I'll explain more fully the diagram.

Keep in mind 1 inch of mercury is a 1000 ft altitude difference as indicated on an altimeter. Forget about temperature right now. It has effects, but you can ignore it for the time being.

In the diagram imagine you are standing on the ground at airport A. To have a correct altimeter setting your altimeter is set to 30.74. That means at 5000 feet the pressure of the air around the aircraft is 25.74 inches. (1 inch = 1000 feet). By setting your altimeter you have calibrated it to read 5000 feet accuratly only if the pressure on the ground stays at 30.74. Like north shore said your altimeter is only a barometer.

Look at airport B now. Imagine that instead of an altimeter setting of 29.74, it is 25.74. That means the yellow line on the diagram is much steeper and now the 25.74 pressure level is actually on the ground. If you don't adjust your altimeter setting to correct, your altimeter will read 5000 feet at the pressure level 25.74. But 25.74 is actually on the ground. If you are in cloud and didn't know any better you would have just followed the yellow line into the ground. (obviously these are extreme cases used for illustration only).

Keep in mind altitude is not the only cause of lower air pressure. Lower air pressure can be caused by areas of rising warm air, among other things. Meaning pressure differences occur at different airports seperated horizontally, but that are the same true height above sea level. As in our example.

[BTD]

From your previous quote I'll add in Blue to show some corrections.

yeah lol I know I am making this to complicated but not sure why!

So okay the stick is at 2500 and we are flying beside it and yes altimeter will say 2500 ASL [correct], now if we go into a low pressure area which means less density [you are confusing yourself by thinking in terms of density. Density does not determine pressure levels. Pressure levels along with temperature determine denisty] which means a increase in the altimeter reading, the pressure level which we used to maintain the 2500 beside the stick is still 2500 ft ASL [no. because the pressure on the ground is now lower, the 2500 ft pressure level from the example is now lower against the stick] but now we are in decreased pressure meaning the stick is below us or yea closer to the ground. [decreased pressure does not mean you have climbed]
right?

I'll add to the example. Thinking standard pressure now of 29.92. This means at 1000 feet the pressure is 28.92 and 2000 ft is 27.92. If the pressure on the ground drops to 28.92 and you don't adjust your altimeter your 1000 ft (28.92) pressure level is now resting on the ground. Your 0 ft pressure level (29.92) is now 1000 feet below the ground and your 27.92 is at 1000 ft above the ground.

Everything has shifted down 1000 ft. Your altimeter only reads 0 ft if you go 1000 ft below the surface of the earth. So on the ground at the airport your altimeter now shows 1000ft. See how thats bad?

I'm starting to talk in circles. So I'll take a break.

[. ._]

I just think of it as "If you fly into a low pressure system and don't adjust your altimeter, you'll crash into a mountain."

That's all I can add. Sorry.

-istp

[justin31]

Okay so lets say airport setting is 29.92 and you go up and the altimeter indicates 26.92 which is 3000 ft. So at airport B the ground altimeter setting is 28.92 which would mean there is a loss of 1000 ft from airport A altimeter setting. If you don't reset the altimeter you are going into a lower pressure area with only a 2000 ft difference instead of the 3000 ft.

So what would the altimeter indicate when you approach the lower pressure? will it indicate a climb so then you will descend and hence the "look out below"

[Stan Darsh]

You are basically correct, although you'll be maintaining your altitude by reference to the altimeter, right? You won't see a "climb" because you are flying the pressure gradient, unless you were flying an exact altitude through GPS or magic. Setting the altimeter in flight allows you to fly at an approximate absolute altitude which is great if you are flying near terrain or obstacles. Otherwise, you're following the ups and downs of the pressure gradient. It sounds like you've grasped the idea.

[loopa]

Now that you understand why the Absolute Altitude changes with a decrease in pressure and failure to reset the altimeter, try and see if you can figure out how flying through a very cold airmass will also give you a lower Absolute Altitude, and why it is you have to correct for it.

And once you get that, throw a bunch of combinations at your self. Such as, what if the pressure is low, but the temp is high? what if the temp is low but the pressure is high? what if the temp is low, and the pressure is low, and what if the temp is high, and pressure is high.

In other words, the density of the air (affected by temperature, and pressure) is what you need to correct for when it comes to the altimeter, for it is calibrated to the STD atmosphere. Just like the Indicated Airspeed Indicator, any deviations from STD will result in corrections needing to be made.

Have fun!

[5x5]

And now that you've got that one straight, work on this one.

Hot to cold, don't be bold.

Same notion, moving from warmer to colder temperature air will result in a misreading on the altimeter to indicate higher than you actually are. There's an altitude correction chart in the AIM RAC section.

[Kabloona]

I would add one more reason for an accurate altimeter setting; traffic. As important as not hitting the ground, not hitting another aircraft. When you give your position report and another pilot indicates they are in the same area remember to confirm their altimeter setting and ensure you're the same. That way (assuming the altimeters are calibrated correctly) you should both indicate correct corresponding indicated altitudes should you use that to assist with seperation.

[172PIC]

Basically if you fly from higher pressure into a lower pressure area your altimeter will think and show the airplane is higher than it really is. When you start your flight set the altimeter to your airports elevation and notice the different pressure reading on it every time.

Also you're paying your flight instructor to teach you - never hesitate to ask any question, no matter how dumb you think it is. It is important to your safety to fully understand everything about flying.

Also it is hard for many to visualize how far a mile is, it takes practice, ask your instructor to quiz you on your flights how far a certain landmark is so you get practice doing this.