SIA_B777
Topic Author
Posts: 467
Joined: Sat Apr 21, 2001 2:28 pm

### What Is FL370?

Sometimes I hear people in this forum talk about FL370.

What does it mean?

hawaiian717
Posts: 3149
Joined: Tue May 18, 1999 2:46 am

### RE: What Is FL370?

It means an aircraft is flying at Flight Level 370, or 37,000 feet.

David / SAN

SIA_B777
Topic Author
Posts: 467
Joined: Sat Apr 21, 2001 2:28 pm

### RE: What Is FL370?

Thanks!

Megatop747-412
Posts: 257
Joined: Tue Sep 19, 2000 1:59 am

### RE: What Is FL370?

SIA_B777

Just to add on to Hawaiian717's note, FL (or flight level) is the term used to denote the altitude above a certain number of feet - I think it is used to descript alt >10,000 feet. Basically, the FL number is the feet minus the last 2 zeros - e.g. FL310 would be 31,000ft...

ben88
Posts: 1037
Joined: Fri Dec 31, 1999 4:49 pm

### RE: What Is FL370?

I believe FL is used at 16,000 feet and above.

Guest

### RE: What Is FL370?

FL370 means, flightlevel 370, 37.000 feet.
Where feet indication ends and flightlevel indication starts, depends on the transition level, which is different in each country and by each airport.
In Amsterdam they have i think the transitionslevel 40, in vienna they have normally transitionlevel 60. It also depends on the local QNH. When the QNH is below 1013Hpa, the transiton level will be higher than normal by 10.

Joona
Posts: 1214
Joined: Sun May 06, 2001 2:02 am

### RE: What Is FL370?

As mentioned above, FL370 means Flight Level 370 and 37000 feet.

There are altitudes and Flight Levels. Then what makes them different? Well, altitude is below and FL is above, but how to know when it changes? Easy: when you go up, altitude changes to Flight Level at Transition Altitude (TA) and when going down, FL changes to altitude at Transition Level (TL). In Finland TA is always 5000ft and TL is FL45-FL65 depending on the QNH. The "standard" TL in Finland is FL55. In the US, TA is always 18000ft.

Just to let you understand this:

1) You are told to climb FL70, TA is 8000ft. Is this possible?

2) You are told to descend to FL55 with TL being 50, possible?

3) Climb 7000ft with TA at 18000ft. Possible?

4) Descend 4000ft, TL 35, possible?

5) Descend FL60, TL 55, possible?

Now think of these questions and see if you can answer them. I have told you everything you need to know.

Joona
1740 days idle. Beat that.

Julien.M
Posts: 146
Joined: Wed Mar 08, 2000 9:22 pm

### RE: What Is FL370?

A Flight Level (FL) is an altitude above a pressure this pressure is always = 1013 HPa. (we call that the Altitude-pressure)
FL370 is 37,000ft over 1013 HPa.

You have to know that pressure is always changing so we use 1013 to be at the same altitude-pressure than other planes...

1 HPa = 30ft.

Imagine you are flying at 3000ft over 1000 HPa and another aircraft is flying in your direction at 3600ft over 1020 HPa. Acctualy you are both flying at the same altitude! both of you are on the isobare 900 HPa. 3000ft + 13 HPa (390ft) = 3390ft and 3600 - 7 HPa (210ft) = 3390ft!
both are flying at 3390ft over 1013 HPa.

But in reality we cannot fly at 3390ft over 1013 HPa... we have to fly some specific FL like 5000ft (FL50), 5500ft (FL55), 6000ft (FL60), 6500ft (FL65)...or 37,000ft (FL370)...that's because we need to have secure altitude separations between aircrafts. (that's the semi-circular rules but I'm not going to explain that it will be too much for the moment).

That can be very confusing when you make a long trip! So the rule say that we have to fly an altitude-pressure (over 1013 HPa) when we are above a certain altitude (in my country when you pass 4500ft climbing over the sea presure [what change all the time] you need to set 1013 HPa on you altimeter...). It's more secure!

I know it can be a bit difficult for people who doesn't fly...Anyway, I hope I'm understandable...
Hope it helps.
Julien from Belgium.
Regards

Joona
Posts: 1214
Joined: Sun May 06, 2001 2:02 am

### RE: What Is FL370?

Ok, there are different abbreviations used in here; I've seen HPa and Hpa...

But the correct is hPa

Joona
1740 days idle. Beat that.

Julien.M
Posts: 146
Joined: Wed Mar 08, 2000 9:22 pm

### RE: What Is FL370?

yes that's right! hPa

Guest

### RE: What Is FL370?

1hPa is 27 feet

TurbineBeaver
Posts: 1511
Joined: Fri Jan 11, 2002 1:50 am

### RE: What Is FL370?

1) NO

2) YES

3) YES

4) NO

5) YES

am i right??????

TB

Joona
Posts: 1214
Joined: Sun May 06, 2001 2:02 am

### RE: What Is FL370?

Turbine:

1) Correct

2) Correct

3) Correct

4) Correct

5) Correct

Now it wasn't that hard was it?   This is a pretty easy thing after you just understand frew facts:

- the meaning of Transition Level and Transition Altitude.

- Difference between Altitude (MSL/AGL) and Flight Level (FL)

And also one thing I forgot to mention is that below TL/TA you always fly according to local QNH / altimeter, and above fly according to QNH 1013 / altimeter 29.91 or 29.92, not sure which one it is.

Joona
1740 days idle. Beat that.

ILOVEA340
Posts: 2064
Joined: Sat Oct 30, 1999 9:49 am

### RE: What Is FL370?

370 is 37,000ft. 39 is 39,000ft and so on.

timz
Posts: 6301
Joined: Fri Sep 17, 1999 7:43 am

### RE: What Is FL370?

I believe SIA_B777 is still scratching his head. Let's try again.

An altimeter is a pressure gauge, right? Somehow, someone has decided that the pressure at 37000 ft altitude is X pounds per square foot, or Y Pascals, or whatever. So they manufacture the altimeter so it reads 37000 ft when the outside pressure is X lb per sq ft. But of course the actual pressure at 37000 ft varies from day to day and from place to place. So if we want to fly at a constant 37000-ft altitude, how do we do it?

The answer is, we don't try to. Instead we just fly along at whatever altitude happens to have a pressure of X lb per sq ft. We call this altitude Flight Level 370. Our actual altitude might be a thousand feet (?) higher or lower than 37000 ft, but as long as all the aircraft up there are following the same rules we'll be okay.

Take a look at http://www.digitaldutch.com/atmoscalc/index.htm

If you key 37000 ft into the input box it will tell you that the "standard" pressure at 37000 ft altitude is 452.438 pounds per square foot, or 21662.7318 Pascals. Those are the "standard day" values of X and Y. So the answer to your question is actually pretty simple: FL370 is the altitude where the actual air pressure, at the moment, is 452.438 lb per sq ft.

For the formulas used to define the "standard day" see http://www.best.com/~williams/avform.htm#Altimetry

sdate747
Posts: 259
Joined: Sun Oct 29, 2000 4:55 pm

### RE: What Is FL370?

What is "HEAVY"? I hear it a lot on ATC and I don't know what it is.

Thanks
Sdate747

XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

### RE: What Is FL370?

An aircraft has "heavy" attatched to its call sign if its Maximum Gross Takeoff weight is over 255,000 pounds. It may or may not be over this weight, it is its maximum certified weight that determines the callsign.
Chicks dig winglets.

SIA_B777
Topic Author
Posts: 467
Joined: Sat Apr 21, 2001 2:28 pm

### RE: What Is FL370?

wow

i am a bit confused, however my understanding for FL370 is pretty clear, it is the hPa stuff that makes me go

Joona
Posts: 1214
Joined: Sun May 06, 2001 2:02 am

### RE: What Is FL370?

SIA_B777, also remember that above Transition Altitude the altimeter / QNH is always set to STANDARD, in this case 1013 hPa or 29.91

Joona
1740 days idle. Beat that.

fxra
Posts: 600
Joined: Fri Jul 16, 1999 1:03 am

### RE: What Is FL370?

STandard pressure is used about a certain height, in the US 18,000 or FL180. At that height, the altimeters are adjusted to a pressure of 29.92. This is the same for all aircraft. SInce the pressure at that atltiude still varies, your may not actually be exactly 18,000 ft above MSL. but since everyone esle is on teh same page it works out.

Below FL180, you must get pressure ratings from WX stations enroute and constantly adjust as needed, or you risk flying into the groud. Especially after bpassing from hi to low pressure areas.

I think i got that right... i hope so, i get to go through the same line of questioning and more tomorrrow with the FAA...

later
Visualize Whirled Peas

Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

### RE: What Is FL370?

The technical standard pressure in millibars is 1013.25 rounded up to the nearest 1/100th mb. An hectopascal is roughly 1/10 of a millibar.

The correct international setting is in fact 1013.25 but on drum style pressure altimeters is accepted at 1013 because the rest is a guess. Airbus uses a push/pull knob simply giving the phrase "STD" for use in the FL's.

"Pressure Altitude" - FL370 or any other FL can be considered more a constant pressure level rather than an actual altitude. Atmospheric pressure in the lower levels (<18,000') has an average decrease of about 1" of mercury in the mercury barometer. Above 18,000, the rate of decrease in pressure is less. Until recently, as aneroid instruments (common altimeter) had variable accuracies in the higher levels, FL spacing in the HL airspace was 2,000'.

With many modern aircraft now flying the NAT's, the Pacific and soon to be continental Europe and North America, most utilizing highly accurate Digital Air Data Computers, vertical spacing is being reduced to 1,000' under a program entitled Reduced Vertical Separtation Minima or RVSM.

Aircraft must meet minimum technical standards in order to participate in RVSM airspace. If they don't, then they are not allowed to fly into it. RVSM was first introduced into the North Atlantic in April of 1997, then in the Pacific in 1999 (I believe). It will be in use over continental North America in early 2002.

Active FL's for RVSM over the Atlantic are from FL285 to FL420 inclusive. Aircraft not certified to fly in RVSM airspace must enter RVSM airspace either underneath or overtop of the limiting FL's. Once continental bodies start participating in the program, flying overtop will be moot as one would have to fly through it to get on top.

Best Regards,

Buff

timz
Posts: 6301
Joined: Fri Sep 17, 1999 7:43 am

### Is That Right, Buff?

Other people are saying a millibar is the same as a hectoPascal. So is it 1/10 after all?

Jan Mogren
Posts: 2014
Joined: Tue Dec 26, 2000 2:47 am

### RE: Is That Right, Buff?

millibar=hPa
just a new name
/JM
AeroPresentation - Airline DVD's filmed in High Definition

Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

### RE: Hecto's Vs Kilo's

Dang metric system! Got them confused. Local weather on the weather channel is in kPa which is to what I mistakenly referred.

Buff was wrong!

Grievous apologies.

Oh well, it'll probably happen again...

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