Reading F) and G) as Vertical Limits

In an ICAO-format NOTAM, F) gives the lower vertical limit of the affected condition, and G) gives the upper vertical limit. These fields describe the vertical extent of an activity, restriction, or other condition. They are not the Q-line code, and they are not the validity times in B) and C). The distinction is essential: F) and G) answer where vertically the condition applies; B) and C) answer when it applies. The Q line is a separate, coded representation used for computer-readable NOTAM information. [1][2]

A NOTAM should therefore be read as several related but distinct kinds of information. The Q line supports standardized coding and digital handling. B) and C) state the start and end of the validity period. D), when present, can specify a schedule within that period. F) and G) state the lower and upper vertical boundaries, while E) contains the plain-language description. These elements complement one another; none should be substituted for another. [1][2]

F) and G) are principally associated with airspace-related NOTAMs, and their presence and interpretation should be considered in the context of the complete notice and applicable procedures. [2] The practical rule is straightforward: locate F) and G), preserve the vertical reference attached to each limit, and read the time fields separately.

A quick example

Consider this illustrative set of fields:

B) 2603291200
C) 2603292000
D) DAILY 1200-2000
F) 1500FT AGL
G) 17999FT AMSL

The B) and C) values identify the validity period; D) gives a recurring schedule. F) and G) describe the vertical band: its lower limit is 1,500 feet above ground level, and its upper limit is 17,999 feet above mean sea level. Because AGL and AMSL use different reference datums, the two numbers cannot be treated as directly comparable figures without considering terrain elevation. [1][2]

That example is not a complete operational interpretation. The NOTAM’s E) text, geographical scope, schedule, and any applicable procedures still matter. The altitude fields define vertical limits, not the full horizontal shape or all conditions of the activity.

How the Vertical References Work

The numbers in F) and G) acquire meaning from their units and reference terms. A value without its datum is incomplete: “1,500 feet” alone does not establish whether the limit is measured above terrain, above mean sea level, or as a flight level. Preserve the complete expression when reading, recording, or comparing the limits. [1][2]

Surface, AGL, AMSL, flight levels, and UNL

SFC or GND indicates a lower boundary at the surface or ground, as specified in the NOTAM.
AGL means above ground level. A limit such as 1500FT AGL is referenced to the terrain below; its corresponding height above mean sea level therefore varies with local terrain elevation.
AMSL means above mean sea level. A limit such as 17999FT AMSL is referenced to mean sea level rather than to the ground directly below the aircraft.
FL, or flight level, expresses altitude in hundreds of feet using the standard pressure reference. It is not interchangeable with an AGL or AMSL value simply because the displayed number appears similar.
UNL indicates that the upper limit is unlimited, where that notation is used. [1][2][2]

These terms are not decorative suffixes. They define how to interpret the value. In particular, an AGL lower limit and an AMSL upper limit may use different reference surfaces within the same NOTAM. If terrain rises beneath an AGL boundary, the corresponding altitude of that boundary above mean sea level rises with it. A reader should not subtract the raw numbers to calculate the thickness of the affected band unless the reference systems have first been reconciled. [1][2]

For example, F) 1500FT AGL G) 17999FT AMSL does not mean a band from 1,500 to 17,999 feet on one common scale. At a location where the terrain is 9,000 feet AMSL, the lower boundary would correspond to 10,500 feet AMSL; over lower terrain, its AMSL equivalent would be different. This is a conceptual illustration of datum conversion, not a replacement for the full NOTAM or applicable operating procedures.

Keep vertical limits separate from validity

B) and C) are time fields: B) gives the beginning of validity and C) the end. D), if included, can describe a schedule during that validity period. F) and G) do not supply those time limits. Conversely, a date and time in B) or C) does not establish an altitude boundary. [1][2]

This separation is useful when a notice includes both a long validity period and a narrower daily schedule. A restriction might have fixed vertical limits while being active only during stated hours. The reader must combine the applicable time information with the vertical band, rather than treating either set of fields as a substitute for the other.

Keep F) and G) separate from the Q line

The Q line includes coded qualifiers, including vertical information intended for standardized processing. Those qualifiers are not the same fields as the textual F) and G) limits. Their values may be converted, rounded, or represented differently for coding purposes, so the Q-line representation need not look identical to the written limits. [1][2][3]

Accordingly, the Q line can help identify or filter a notice, but its encoded vertical qualifiers should not be used as a replacement for reading F) and G). If the representations appear different, retain the distinction and review the full notice and relevant procedures rather than assuming the values are interchangeable. The purpose of this distinction is not to disregard the Q line; it is to avoid confusing a coded representation with the separately stated lower and upper limits.

Reading Methods: Human Review and Digital Processing

A disciplined reading sequence works for both manual review and software-assisted workflows:

Find F) and G). Identify the lower and upper limits as written.
Preserve each datum and unit. Note whether the value is SFC/GND, AGL, AMSL, a flight level, or UNL.
Check whether the datums differ. Do not compare unlike reference systems as if they shared a common scale.
Read B) and C) separately. Establish the validity period, then check D) for any schedule.
Treat the Q line as a separate coded representation. Do not substitute its qualifiers for the published F) and G) limits.
Review the complete NOTAM. E) and the notice’s other information may be necessary to understand the affected condition and its scope. [1][2]

Manual interpretation makes it possible to notice context that a simple text extraction could miss, such as unlike datums or a schedule that narrows when the notice is active. Software can support search, sorting, and display, but reliable handling depends on keeping field meanings distinct and retaining the original text for review. This is a general design consideration, not a claim that every system parses or displays NOTAMs in the same way.

VectorWX is a student tool for reading these fields. Whether the message is read as text or in an interface, the distinction is the same. F) and G) state the textual lower and upper limits; the Q line provides a separate coded representation; B) and C) provide validity times. [4] An interface may help organize information, but a reader should still verify the original fields and their reference datums.

Long-Term Implications for NOTAM Interpretation

NOTAMs combine structured labels with text that readers and systems must interpret. That format makes field discipline important: a value can be correctly extracted yet still be misunderstood if its datum is dropped, or if a coded qualifier is mistaken for the textual limit. The operational challenge is therefore not only to find altitude numbers, but to preserve what each number refers to and how it relates to the rest of the notice.

As aeronautical information becomes more digitally processed, the distinction between human-readable text and machine-readable representations remains consequential. Standardized coding can support search and filtering, while the textual fields retain specific limits that must be read in their stated context. The available FAA guidance and NOTAM material describe these roles separately; they do not justify treating the Q-line limits as a direct replacement for F) and G). [1][2][3]

For readers, the durable habit is simple: interpret F) as the lower vertical limit and G) as the upper vertical limit, retain each stated datum, and keep both separate from the Q-line code and the B)/C) time window. Then use the full NOTAM and current applicable procedures for operational decisions. This prevents a basic category error—confusing altitude with time or a coded qualifier with the textual vertical boundary—without overstating what the altitude fields alone can establish.