A small question about the LEGS page

[Sorux123 0]

Hello forum. In the LEGS page on the CDU, im wondering what these different lines mean(they are just examples) 1) 250/FL0872) 267/FL147A3) 236/098A4) 240/010So i particular what the FL's, and A's refer to. And i guess there could be B's as well, but ive only seen that for the speed. Thanks in advance!

[sgreen91 142]

FL= Flight Level dependent on transition altitude as defined in FMCA = Be at or above this altitude at this waypointB = Be at or below this altitude at this waypoint. Hope this helps.

[Sorux123 0]

Ah ok. But what exactly does "Flight Level dependent on transition altitude as defined in FMC" mean? How about the lines that are written with thicker letters? Doesent that also indicate a restriction?

[Spin737 102]

How about the lines that are written with thicker letters? Doesent that also indicate a restriction?

There are four options: Big Numbers: Be at this altitude at the fixFollowed by an A: At or AboveFollowed by a B: At or BelowLittle numbers: FYI. The FMC thinks it'll be around this altitude when it crosses said fix

[Flexman 30]

Ah ok. But what exactly does "Flight Level dependent on transition altitude as defined in FMC" mean? How about the lines that are written with thicker letters? Doesent that also indicate a restriction?

Is the term "flight level" what you dont understand here?

[EmileB 12]

Is the term "flight level" what you dont understand here?

Flexman21, just in case that Sorux123 (or someone else) indeed does not understand the term flight level, this is my simple explanation as a layman (so anyone please correct me if I'm wrong) :In some parts of the world, aircraft altitudes (above mean sea level or MSL) are given in metres but in most of the world they are given in feet (one foot is about 0,3 metre). Below transition altitude, aircraft altitude is expressed in feet and above transition altitude it is expressed in flight level. Flight levels are given in hundreds of feet. For example : 24500 ft is FL245. Transition altitude is not the same all over the world. For example : in the US it is 18000 ft, here in New Zealand it is 13000 ft, in the UK 6000 ft (I think). As Sean Green already said, the correct transition altitude for the airspace in which one flies, can be programmed into the FMC via the CDU. So think of flight levels as a kind of shorthand for altitude ! Hope this helps.

[Sorux123 0]

Your responses are greatly appreciated, helped me alot:) Thanks for the great tips!

[Flexman 30]

Flexman21, just in case that Sorux123 (or someone else) indeed does not understand the term flight level, this is my simple explanation as a layman (so anyone please correct me if I'm wrong) :In some parts of the world, aircraft altitudes (above mean sea level or MSL) are given in metres but in most of the world they are given in feet (one foot is about 0,3 metre). Below transition altitude, aircraft altitude is expressed in feet and above transition altitude it is expressed in flight level. Flight levels are given in hundreds of feet. For example : 24500 ft is FL245. Transition altitude is not the same all over the world. For example : in the US it is 18000 ft, here in New Zealand it is 13000 ft, in the UK 6000 ft (I think). As Sean Green already said, the correct transition altitude for the airspace in which one flies, can be programmed into the FMC via the CDU. So think of flight levels as a kind of shorthand for altitude ! Hope this helps.

I'll add a little to that. As you say, FL245 refers to an altitude of 24500 feet, but that is pressure altitude. Flight levels are isobaric layers (constant pressure levels) at which traffic arranges itself for standarized altimetric references. For example, FL180 is at whatever true altitude at which the pressure is 500hpa. Some factors affect flight levels:Temperature: On hotter than ISA days, flight levels are further apart from eachother. On colder days, flight levels are closer to eachother (typical test question: What is the vertical distance between an airplane flying at FL200 and another one flying at FL220 on ISA+15 conditions. Hint: It is more than 2000ft) Pressure:On days with higher than ISA pressure, all flight levels are lower. On low pressure days, flight levels are higher. Altitude: The pressure gradient is affected by the altitude. At lower levels, 1hpa corresponds to 27ft. Above 18000ft, 1hpa corresponds to 50ft. That means that higher flight levels are further apart than lower flight levels. This is not only affected by the current time conditions, but also for the local conditions. That means that, an airplane flying along a flight level (that is, flying at constant pressure altitude), it climbs and descends depending on the atmospheric presure and temperature of the air mass that it flyes through... and so do the other traffics, so as long as we set 1013,2 in the collsman, we're fine. So, why do we use altitudes at lower levels and flight levels when flying high? Well, sometimes we care more about where the ground is, and sometimes we just want to be far from other airplanes. It is often said that flight levels are those read at the altimever when it is set to QNE. That is actually wrong. QNE is the actual indication of the altimeter, when it is set to 1013,2hpa. So, it is correct to say that at FL200, your QNE is 20000ft. QNE is the Qcode for "pressure altitude".

[JC75 22]

Flexman Id have thought youd have put the little pciture in to of the plane flying over the sea and land! lol

[EmileB 12]

I'll add a little to that. As you say, FL245 refers to an altitude of 24500 feet, but that is pressure altitude. Flight levels are isobaric layers (constant pressure levels) at which traffic arranges itself for standarized altimetric references. For example, FL180 is at whatever true altitude at which the pressure is 500hpa.

Thanks a lot for the additional explanations : one learns something every day ! I've copied the text of your reply and saved it in a document on my PC for future reference.

[Flexman 30]

Flexman Id have thought youd have put the little pciture in to of the plane flying over the sea and land! lol

Hehe. You know, flight levels don´t really care if you fly over sea or land. You might be confusing them with the difference between altitude and height. But i have a better example for showing flight levels graphically. I´ll leave it for tomorrow though

[BillCusick 75]

Hi Flexman.. << I'll add a little to that.>> Oh heck. Here's me, just about got my head around the FMS, and now you come herewith your 'I'll add a little to that' and my brain hurts. I'll just fill in the odd alt correction on the FMS legs. I'm ready to give up !I'm suffering with data overload, ever since this aircraft was released... Bet you haven't taken 'Global Warming' into account.. Ha !Bill Cusick

[Flexman 30]

Hi Flexman.. << I'll add a little to that.>> Oh heck. Here's me, just about got my head around the FMS, and now you come herewith your 'I'll add a little to that' and my brain hurts. I'll just fill in the odd alt correction on the FMS legs. I'm ready to give up !I'm suffering with data overload, ever since this aircraft was released... Bet you haven't taken 'Global Warming' into account.. Ha !Bill Cusick

Well, then be prepared. During pilot training I used to spend hours in student pilot forums and I was the guy who would make diagrams and stuff for explaining things the way I undestood them. It's been a long time since i dont talk aviation basics, so be prepared for some more "little additions". Before training, I used flight simulator since i was probably 10 years old, and it is normal to learn something at 0.1% of what it really is. Like, flight levels? Yeah, a new way to notate altitude. Then you get to train and you see that there is sooo much more to it. Grab another concept, for example. Flexible take-of procedures. V1, stick shaker... any of those things have pages and pages of explanations and regulations. It's a lot of info, but it is beautiful info. So, for me, the NGX and this forum have become a reason to get my old ATPL books and to start refreshing long forgotten things. What better way than explaining it to those who might need it for getting the most out of flight simulator? Cheers and have fun.

[Mats_J 1]

I'll add a little to that...

Some great additions there. I learnt something new today! Thanks,

[JC75 22]

Hehe. You know, flight levels don´t really care if you fly over sea or land. You might be confusing them with the difference between altitude and height. But i have a better example for showing flight levels graphically. I´ll leave it for tomorrow though

Oh dont worry I know all the differences between Height, altitude, and FL (im the one that clears you to them all!)- was just expecting the standard diagram that they show when teaching this stuff of the little plane flying along over the sea and land and then all the lines depeciting the points based on which pressure setting and what it gives you in reference to height, altitude and FL- then the fun of working out TA/TL and layer - which becomes an even more confusing subject lol Then we can have the bit on modifying it for temp, low/high pressure