Re: Malaysian Airlines Boeing 777 (MH370) goes missing Quote:
Originally Posted by radek Jeroen, What is the meaning of terms like FL350, FL295 etc? |
See the answer below Quote:
Originally Posted by saket77 FL is abbreviation of Flight Level, which in layman terms mean altitude of the aircraft. It is normally conveyed in hundreds of feet. Hence FL350 means an altitude of 35,000 feet.
FL as I said is altitude in layman terms but altimeter in planes is only a calibrated barometer or a pressure altimeter, hence it is also specific barometric pressure. Hope I have not confused you. I manage to do it with even intelligent guys around
Edit: And sorry to bump up a question meant for Jeroen. |
No problem and your answer on what it means is spot on. How it is measured needs a little further explanation, because Flight Levels are given against a set barometric pressure of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's true altitude.
Let me explain in what I hope are simple terms. For obvious reasons it is necessary to get an accurate way determining altitude for each and every plane in the sky. The first thing you need to do then is to agree a datum, i.e. what do you call 0 (zero). In aviation (and in other fields as well) altitude is measured against Mean Sea Level (MSL). Altitude above MSL can be easily measured as indicated before by measuring the air pressure. Essentially the further you are away from zero the lower your reading will be. However, you do need to adjust for the actual local barometric pressure. Again, I'm simplifying here. When you're flying at relative low altitudes you need to constantly adjust your altimeter for actual barometric pressure. These values are transmitted constantly as part of the ATIS messages on the radio, or Air Traffic Control will be able to tell you. It is very important to do this often and set your altimeter accurately. Before you know it you can introduce and error of hundreds of feet.
If you want more details/ information have a look here: https://www.faa.gov/regulations_poli...apter%2007.pdf
On long hauls, and high up, the constant adjustment of the altimeters is not practical so once you climb above a certain altitude you will switch to what is known as pressure altitude or sometimes also referred to as standard altitude. What you do is you dial in a set (standard) pressure. The altitude where you change over is known as transition altitude. From there upwards you call out the altitudes as Flight Levels
Above the transition altitude everybody sets their altimeter to 1013.25 hPa (29.92 inHg). This will ensure vertical separation. Below the transition altitude, as explained you need to constantly adjust your altimeter for actual barometric pressure. This will do two things. It will ensure vertical separation between planes, and it will tell everybody their altitude above ground. Aviation charts typically show altitudes as above mean seal level. So for instance Delhi airport is at 773 ft above sea level. So with the plane on the ground and the correct barometric pressure set the pilots altimeter will read 773 ft.
The transition altitude is different around the world. In the USA it's (mostly I think) 18000 feet, where as in Europe it varies from a couple of thousand feet upwards.
Have a look for some more detail: http://en.wikipedia.org/wiki/Flight_level
So, a flight level will ensure vertical separation it does not indicate how high up you are from the ground or from MSL. But at Flight Level altitudes that is of no relevance.
You will notice we are talking about feet, there are a few countries out there that are using metric measurements. So they will have altitudes expressed in meters and Flight Level in hundreds of meters. Altimeters of most commercial airliners have an option to simply switch the scale of the altimeter from feet to meter and vice versa.
Hope this help,
Jeroen |