Under Pressure (the Sea Level type)

[JRSLim]

I have a question about Sea Level Pressure.
I understand what it is as being "A pressure value obtained by the theoretical reduction of barometric pressure to sea level -Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station. "

But what I don't understand so much is -- if I know the station pressure/pressure altitude, What does knowing SLP help me determine?

Thanks in advance
Shaun

 

[tarp]

Hmmm. Life at 6,184 elevation or above Mean Sea Level (MSL).

How about this? How far would you have to fly to fly your C-172 or PA28 at 2,500MSL?

Death Valley? Kansas?

If an airplane is flying at 8500'MSL over Atlantic City, NJ (ACY) how far above the ground/water is it? Just about 8500', right?
So it flies west heading towards COS. It gets over Cincinnati, OH still at 8500MSL and ATC say to look for traffic 7500'agl of the airport. Is that a concern? - Well yes, because that airplane is also at 8500'MSL and in fact ATC would never say look for traffic at 7500agl - they would say - traffic, 12 o'clock, five miles, also at eight thousand, five hundred.

When the airplane arrives at COS (and since this airplane is setting a record of most stable aircraft in history holding 8,500'), the pilot finds that he/she is now only 2,300ft agl but traffic at 8500MSL is still a concern.

Ergo, MSL is a datum just like the CG Arm in your airplane's W&B calculation is a datum plane. It is a theorectical tick mark where everything else is measured from. It would be chaotic for everyone to have to translate height above the ground to keep track of where everyone is. You could be over Pike's Peak and say you are 500'agl but that means a turbo prop flying 15,000MSL has to worry about you. The same could be said for Cessna and Piper - just think if they had to issue charts in CG inches for every airplane manufactured with all the different options and radios. With a datum plane, they can adjust a "standard" chart for your airplane.

In most airplanes, the "tick mark" is set at the firewall, the prop or some line forward of the prop. In MSL, the line is set at Sea Level. How many places on the planet can you land your airplane below sea level? Well there is Death Valley - and its kinda cool to see your altimeter at less than zero. But the answer is not too many.

 

[firstthird]

I like to think about it in reference to a "Standard Day." Airplanes have their performance numbers measured in reference to this 'standard day.' That being at sea level and 59 F or 15 C. If you are higher or hotter, then you make corrections. There is more to it than that but that is why you care. In Colorado you'll get less push from your engines and have a higher TAS in relation to IAS, meaning you'll be using more runway to get airborne and land than you would with the same temperature at sea level.

I agree though that we do everything in MSL when flying for the ease of use, which is also whey we go to 29.92 above FL180 in this country (since it wouldn't be easy or practical to ask for a local altimeter every 100 miles on a transcon). In Europe they have a Transition Altitude much lower, like 7000 or as low as 4000 feet in some places and there it can very with a very low or high atmospheric pressure so they put the transition altitude/level on the atis.

 

[JRSLim]

Let me clarify

Tarp

Yeah I guess it might seem like I'm asking an explainantion for a basic aviation procedure, and I understand that sea level is the datum for pressure altitude, and I am familiar with using MSL altitudes (yes we do there here in the high country) I do know that much at least.

But What I was trying to clarify is -- If I know the altimeter setting for my local area, set my Kohlsman to that and am able to see what MSL I am at, what relevance does the slp have in the metar? Is it redundant at that point, or if I know slp and intend to fly from Miami to Colorado is it then I look at SLP instead of listening to the local atis to figure out? Because whatever the SLP reading is, the Altimeter setting will be proportional correct?

Thanks for the input
Shaun

 

[Mr. Cole]

If I'm not mistaken, and I may well be, the pressure you receive from the ATIS is the local pressure corrected for Sea Level. In other words, say the field is at 1000' agl and the pressure reading at the field is 27.92. When you tune in the ATIS, the pressure you receive will not be 27.92, but something like 28.92, assuming 1000' is about 1" of Hg. As you mentioned in your first post, there are two knowns at the measuring station, the local pressure at the field elevation and the field elevation itself.

Therefore if you're reading 27.92 at 1000', then you can extrapolate what the Sea Level Pressure Altitude must be in order to give you that observed reading at your field elevation. Again this calculation assumes that the atmospheric properties are the same in a column of air extending hypothetically from the field down to sea level. This reading should be exactly equal to pressure altitude at standard temperature.

But let's say you've set your altimeter at 28.92 and you go flying for a long distance and don't reset you altimeter often. Let's also say you're flying to an area of lower pressure. While you're true altitude has not changed, the altimeter will interpret the decrease in pressure as an increase in altitude, although you really haven't gone anywhere. So your altimeter will read a higher indicated altitude than the true altitude. Again, high to low, look out below.

Hope I'm somewhat right and I hope it helps.

Dave