Reading a Light's Characteristic: Light List, Period and Range

What the rule requires

A light is described by its characteristic, colour, period, height and nominal range, in that order. Fl R 4s is a red light flashing once every four seconds Bowditch Ch. 5 §503. Read the fields in that order every time and the abbreviation decodes itself; read them out of order and you will take the period figure for something else.

The Light List is the authority for the character, range, height and structure of every aid, and it is the reference for identifying a light by its characteristics at night. It is the authoritative reference for USCG-maintained aids, and it must be updated against the weekly Local Notice to Mariners Bowditch Ch. 5 §502. On an open-book exam the figure is in front of you; the point being tested is whether you know which column and which publication answers the question.

Period is the whole cycle

Fl(2+1) R 6s is two red flashes, a brief eclipse, one red flash, then a long eclipse, repeating every 6 seconds Light List — light-characteristic (Fl(2+1)). The 6 seconds covers all three flashes, both eclipses and the long dark interval. It is not the interval between flashes and it is not the length of the group.

A charted light reads `Fl(4) 15s`. How long from the first flash of one group to the first flash of the next?

Fifteen seconds. Fl(4) is four flashes per group Light List — light-characteristic (Fl(4)), and the period figure is the time for the whole cycle to repeat, so all four flashes and the following eclipse fit inside the 15 seconds.

The characteristic tells you what kind of mark you are looking at

• Mo(A) — a short flash followed by a long flash, repeating; used on safe-water marks indicating a fairway entrance Light List — light-characteristic (Mo(A)). The safe water mark itself is a red-and-white vertically striped buoy marking deep navigable water, often the seaward end of a channel, lit Mo(A) W Light List — Safe Water Mark.
• LFl — a single flash of two seconds or more, often used on safe-water or fairway marks Light List — light-characteristic (LFl).
• Fl(2+1) — group-flashing patterns separated by a longer eclipse, used on preferred-channel buoys.
• Fl(2) — two flashes per group repeated at a fixed period, common on starboard lateral marks in US waters with 5- or 6-second periods Light List — light-characteristic (Fl(2)).
• Oc — a continuous light interrupted by regular short eclipses, the duration of light longer than the duration of darkness; the opposite of flashing Light List — light-characteristic (Oc). At a distance the eclipses register as the events, which is why an occulting light gets reported as flashing.
• F — a continuous, steady light, used on minor harbour entrance lights and private aids Light List — light-characteristic (F).
• Q and VQ — the two rapid classes, separated below.

Isophase (Iso) sits alongside fixed, flashing, occulting and quick as one of the common charted characteristics. Where a question turns on what an isophase light looks like, the Light List entry for the aid settles it.

The colour letter

The colour follows the characteristic, as the R does in Fl R 4s, and it is written out where it matters: the safe-water mark is listed Mo(A) W. Yellow is used on special-purpose marks, towing lights and anchorage area markers Light List — light-characteristic (Y). The Rules specify chromaticity standards for the four navigation-light colours — white, green, red and yellow — by CIE co-ordinate 33 CFR §84.13.

Sectored lights

A sectored light shows different colours in defined arcs of bearing, and the mariner's bearing through the sectors indicates safe versus danger water Light List — light-characteristic (sec). A lighthouse charted as flashing white with a red sector is telling you that while you hold the red arc you are looking across danger; the sector limit is a bearing, not a channel edge. This appears on the exam almost verbatim, and the distractor is always "it marks the limits of the navigable channel."

Range

Range in the Light List is a property of the aid and its structure:

• Major light — high-intensity, shown from a fixed structure, visible 15+ nm; used for landfall or coastal navigation Light List — Major Light.
• Secondary light — a coastal light of moderate intensity, typically 10–15 nm, from a smaller structure than a major light Light List — Secondary Light.
• Minor light — a small fixed light on a pile or platform marking harbour entrances or channel features, typically 5–10 nm Light List — Minor Light.

One more Light List field earns its keep at night in poor visibility: a RACON responds to your radar pulse with a coded Morse character painted on the display, and the Morse code is given in the Light List Light List — RACON.

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Telling it apart

The confusable set here is the fast-flashing family, and the criterion is flashes per minute plus whose light it is — an aid to navigation or a vessel.

• Quick flashing (Q) — 60–80 flashes per minute, used to indicate cardinal points internationally or hazards on the US Western Rivers Light List — light-characteristic (Q). Misfiled as "any fast light."
• Very quick flashing (VQ) — 80–160 flashes per minute, which distinguishes cardinal marks from quick-flashing aids Light List — light-characteristic (VQ). The north cardinal mark carries two black cones both pointing up, black above yellow, lit VQ or Q white, and you pass to the north of the danger Light List — North Cardinal Mark.
• Flashing light, Rule 21 sense — a light flashing at regular intervals at a frequency of 120 flashes or more per minute 33 CFR §83.21. This is a definition for lights carried by vessels. It does not set the rate for a charted Fl aid, and candidates who carry the 120 figure into a Light List question answer confidently and wrongly.
• Special flashing light — a yellow light flashing at 50 to 70 flashes per minute, placed as far forward and as nearly as practicable on the fore and aft centreline of the tow, showing over an arc of not less than 180° and not more than 225°. A towing light, never an aid characteristic.
• Group flashing, Fl(2) and Fl(4) — the figure in parentheses counts flashes per group. It says nothing about rate, and nothing about the period, which is stated separately in seconds.

You are told a light shows 90 flashes per minute. Is that quick flashing, very quick flashing, or a "flashing light" as the Rules define it?

Very quick flashing: 90 falls inside the 80–160 per minute band for VQ, above the 60–80 band for Q, and below the 120-or-more threshold the Rules use to define a flashing light on a vessel. The Rule 21 figure has no bearing on which class of aid characteristic this is.

Working a question

You are inbound at night and the chart shows a lighted buoy annotated Fl(2+1) R 6s. Identify it and confirm it.

1. Split the annotation into its fields. Characteristic Fl(2+1), colour R, period 6s. No height or nominal range is printed on the buoy annotation; those come from the Light List entry.
2. Decide what the parentheses count. Composite group flashing: two flashes, a brief eclipse, one flash, then the long eclipse. Three flashes in the cycle, unevenly spaced. If you had read it as Fl(3) you would be looking for three evenly spaced flashes and would reject the correct buoy when you saw it.
3. Name the mark. Composite group flashing on a red light is a preferred-channel buoy. Had the annotation read Fl(2) R 6s you would be looking at a starboard lateral mark instead, and the two differ on paper by one character.
4. Time it to confirm. Start the watch on the first flash of the group and stop it on the first flash of the next group. Six seconds confirms the aid. Three seconds means you stopped on the single flash inside the cycle and timed a fragment, not the period.
5. Go to the Light List for what the chart does not carry. Character, colour, period, height, nominal range and structure are there, and the entry is the authority if the chart annotation and what you see disagree.
6. Check the correction state before you rely on it. An aid whose characteristic was changed since your Light List was printed is carried in the weekly Local Notice to Mariners. An uncorrected volume gives you a confident, obsolete answer.

Where candidates lose the point

• Reads the period as the flash interval. Given Fl(2+1) R 6s, the candidate answers that six seconds separates the flashes. The 6 s is the full cycle: two flashes, brief eclipse, one flash, long eclipse, repeat.
• Imports Rule 21 into a Light List question. The Rules define a flashing light at 120 or more flashes per minute, so a candidate asked about a quick-flashing aid reaches for 120 instead of 60–80. Rule 21 defines lights carried by vessels.
• Confuses an aid's nominal range with Rule 22 minimum visibility. 33 CFR §83.22 sets minimum ranges for lights carried by vessels — for a vessel of 50 metres or more, a masthead light 6 miles and a sidelight 3 miles — and those figures belong to vessel lights. A light's range in the Light List is a property of the aid: 15+ nm for a major light, 5–10 nm typically for a minor light. Answering "3 miles" for an aid because the figure appeared in Rule 22 is a wrong answer that felt sourced.
• Calls an occulting light flashing. The eye reads the short eclipses as the events. The test is the ratio: in an occulting light the duration of light exceeds the duration of darkness, which is the reverse of flashing.
• Takes a red sector for a channel boundary. It marks danger water on that bearing. The sector limits are bearings and the arcs are defined in the Light List.
• Mixes the colour letter with the numbering scheme. 33 CFR §62.43 governs numbers and letters carried on the aid: red aids bear even numbers and green aids bear odd numbers, increasing in the Conventional Direction of Buoyage, and where light-reflecting material is fitted, letters and numbers may be white. White retroreflective digits on a red buoy do not make it a white light.

Check yourself

You sight a buoy showing a short flash followed by a long flash, repeating, in white. What have you found, and what does it tell you about the water around it?

Mo(A) W on a safe-water mark — a red-and-white vertically striped buoy marking deep navigable water, commonly the seaward end of a channel. Deep water is available on all sides; it is not a lateral mark and you do not pass it on a prescribed side.

A lighthouse is charted flashing white with a red sector. What is the red sector for?

It marks a danger area. A sectored light shows different colours over defined arcs of bearing, and your bearing through the sectors is what tells you whether you are in safe or dangerous water. It does not mark the limits of the navigable channel and it is not there to help identify the light.

Two entries read `Fl(2) R 6s` and `Fl(2+1) R 6s`. Same colour, same period. What is different on the water, and does it change how you handle the buoy?

Fl(2) is two flashes per group, evenly repeated — common on starboard lateral marks in US waters at 5- or 6-second periods. Fl(2+1) is two flashes, a brief eclipse, then a single flash before the long eclipse, and that composite pattern marks a preferred channel. The extra flash after the short eclipse is the whole distinction, so count within the group before you decide which buoy you have.

You are looking for a landfall light on a coastal passage and the Light List entry shows a nominal range of 12 nm from a modest structure. Is this the light you want?

That is a secondary light, moderate intensity in the 10–15 nm band from a smaller structure. A major light is the landfall aid: high intensity from a fixed structure visible 15+ nm.

A light you have been running on has changed its characteristic since your Light List was printed. Where is that carried, and how often?

The weekly Local Notice to Mariners. The Light List is the authoritative reference for the characteristics, range, height and structure of USCG-maintained aids, and it is only authoritative when kept corrected against the Notice.

Your tow carries a yellow light forward flashing at about 60 per minute. Is that a quick-flashing aid characteristic?

No. That is a special flashing light: yellow, 50 to 70 flashes per minute, as far forward as practicable on the fore and aft centreline of the tow, showing over an arc of not less than 180° and not more than 225°. Quick flashing (Q) at 60–80 per minute is a characteristic of an aid to navigation, used for cardinal points internationally and for hazards on the US Western Rivers. The rate bands overlap; the vessel-versus-aid distinction is what answers the question.