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What is the difference between "S" and "M" in flight plan altitudes?

The FAA's ICAO flight plan documentation says:

Standard Metric Level in tens of meters, expressed as S followed by 4 figures (for example, S1130), or

[...]

Altitude in tens of meters, expressed as M followed by 4 figures (for example, M0840)

I don't know what "Standard Metric Level in tens of meters" means exactly? How is that different from "Altitude"?

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    $\begingroup$ Does this answer your question? What is the difference between "flight level" and "altitude"? $\endgroup$ – expeditedescent Jun 13 '20 at 5:24
  • $\begingroup$ The terms asked about here aren't used in that question. $\endgroup$ – Ralph J Jun 13 '20 at 13:39
  • $\begingroup$ @RalphJ Flight level and Altitude have exactly the same meaning, the only difference is the unit (metres vs. feet) $\endgroup$ – expeditedescent Jun 13 '20 at 18:43
  • $\begingroup$ @expeditedescent - I think you meant to say, Standard Metric Level and Flight Level “...have exactly the same meaning, the only difference is the unit (metres vs. feet)“. $\endgroup$ – Dean F. Jun 13 '20 at 18:47
  • $\begingroup$ @DeanF. No, what I mean to say is that flight level is called flight level and altitude is called altitude, no matter if you are using feet or metres as the unit $\endgroup$ – expeditedescent Jun 13 '20 at 19:34
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Standard Metric Level is the metric equivalent of the foot-based Flight Level. It is what you get when the altimeter is set to QNE, i.e. above the Transition Level, and read in meters.

Metric Altitude is the metric equivalent to the foot-based Altitude. It is what you get when the altimeter is set to QNH, i.e. below the Transition Altitude, and read in meters.

These are actually the official ICAO systems, with the foot-based systems declared a "temporary" alternative.

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In the FAA's ICAO flight plan documentation, it states the following:

Cruising level (maximum 5 characters)
ENTER the planned cruising level for the first or the whole portion of the route to be flown, in terms of:
Flight level, expressed as F followed by 3 figures (for example, F085; F330), or
*Standard Metric Level in tens of meters, expressed as S followed by 4 figures (for example, S1130), or
Altitude in hundreds of feet, expressed as A followed by 3 figures (for example, A045; A100), or
Altitude in tens of meters, expressed as M followed by 4 figures (for example, M0840), or
for uncontrolled VFR flights, the letters VFR.

Flight Level and Standard Metric Level are synonymous terms for the same value. Flight Level is measured in imperial feet. Standard Metric Level is measured in metric meters. Either, whether represented in feet or meters, is the distance at which an aircraft flies above the atmospheric level of standard pressure (Pressure Altitude). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to 29.92 inches of mercury (or the metric equivalent). In the US, Flight Level is what you would use at and above FL180.

Altitude, whether represented in feet or meters, is the distance at which an aircraft flies above the standard datum line of the average level of the sea (Mean Sea Level). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to the local atmospheric pressure (if it is accurate). In the US, altitude is what you would use below 18,000 feet MSL.

Depending on atmospheric pressure, there will be a transition space between 18,000 feet MSL and FL180.

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  • $\begingroup$ The question was about "Standard Metric Level", not "Flight Level". $\endgroup$ – HiddenWindshield Jun 13 '20 at 16:13
  • $\begingroup$ @HiddenWindshield - How are Standard Metric Level (represented in meters( and Flight Level (represented in feet) not, essentially, the same thing? The question does not ask for the difference between Standard Metric and Flight Level. The OP already understood that per his original, unedited, question. The OP is asking the difference between Level and Altitude. My answer (as it states) answer the question of that difference whether represented in metric meters or imperial feet. $\endgroup$ – Dean F. Jun 13 '20 at 18:47
  • $\begingroup$ In the original, unedited question (as you put it), user50042 never mentioned Flight Levels, so there is no evidence they understood that "Flight Level" is equivalent to "Standard Metric Level". All subsequent edits were done by different users (specifically, Pondlife and expeditedescent) so there's still no evidence that user50042 knew this. You can't just assume that the asker is going to know that the two terms are equivalent, especially with such a basic question. $\endgroup$ – HiddenWindshield Jun 13 '20 at 20:25
  • $\begingroup$ @HiddenWindshield - Pondlife specifically added to the references to Flight Level to the question. The OP did not mention Flight Level. I answered the question as it was asked after Pondlife’s edit. That is the issue with edits. Unless you play detective with each question, you can’t be sure the edit has the original intent of the OP. $\endgroup$ – Dean F. Jun 13 '20 at 20:42
  • $\begingroup$ Yeah, that's true. $\endgroup$ – HiddenWindshield Jun 15 '20 at 3:12

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