What is the speed of the approach lights rabbit?

Question

If you were flying past the rabbit and passed each bulb as it lit, what speed would you be flying at? The rabbit is 2400 feet long, spaced at 100 foot increments and consists of 15 lights. The math doesn't add up on that but it's here on the FAA Visual Guidance Lighting Systems. It flashes twice per second but it's not as simple as 2400 feet in half a second, because the lights don't immediately start over when it reaches the end. So the simulated speed is a bit faster than 3272 mph (2400 ft / 0.5 sec).

https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/lsg/als

Answer 1

There are several different systems of Sequenced Flashing Lights, which is the official name for the "rabbit." Generally these flashing lights are spaced 200 feet apart.

FAA Order JO 6850.2B is the current specification for "Visual Guidance Lighting Systems." It describes the following systems which include flashing lights:

• ALSF-2. Fifteen flashing lights spaced at 100-foot intervals from THR–1000' to THR–2400' (total: 1400').
• SSALR. Five flashing lights spaced at 200-foot intervals from THR–1600' to THR-2400' (total: 800').
• MALSR. Five flashing lights spaced at 200-foot intervals from THR–1600' to THR-2400' (total: 800').
• MALSF. Three flashing lights spaced at 200-foot intervals from THR–1000' to THR–1400' (total: 400').
• ODALS. Five flashing lights spaced at 300-foot intervals from THR–300' to THR–1500' (total: 1200').

All of the above systems are described as "flash[ing] in sequence toward the threshold at the rate of twice per second" except for ODALS, which is not described as flashing toward the threshold at all; however from watching a YouTube video depicting an ODALS installation and timing with a stopwatch, I judge that ODALS flashes at a "rate" of once per second. (You can compare this to a video of an ALSF-2 and see the difference.) But this tells us only how often the sequence begins; it does not tell us how long the sequence takes (although there is an upper bound of 0.5 seconds, as there is only one sequence active at any one time).

Your question asks: What is the speed at which the "flash" travels down the length of the SFLs? We can answer this definitely if we know two pieces of information: the distance between successive flashers, and the time interval between successive flashes. We know the first but we do not know the second.

As @abelenky describes, we can assume that the sequence takes the entire 0.5 seconds to elapse, in other words, that it takes the rabbit the entire 0.5 seconds to travel 1400 feet:

$$ \frac{1400 \mathrm{~ft}}{0.5 \mathrm{~sec}} \cdot \frac{1 \mathrm{~NM}}{6076 \mathrm{~ft}} \cdot \frac{3600 \mathrm{~sec}}{1 \mathrm{~hr}} = 1650 \mathrm{~kt} $$

This gives us a correct lower bound on the rabbit's speed for the ALSF-2. If we performed this calculation on the MASLR or MALSF we would reach a lower lower bound, because we would be assuming that the rabbit takes the entire 0.5 seconds to travel 800 feet or 400 feet respectively. But my strong suspicion is that the rabbit travels at the same speed no matter the type of system, and the only difference is the "pause" at the end of each sequence—a shorter system will complete its sequence in less time and require more of a post-sequence delay until the next half-second.

The definitive answer can only come from FAA documentation describing the light-to-light timing, and I have not been able to find that easily. I will ask around and see what I can do. In the meantime, I have taken the ALSF-2 YouTube video from above and stepped through it frame by frame; I can confirm that it takes twelve frames (0.5 seconds) for the sequence to re-start, and further that it takes only seven frames for the sequence to elapse. Thus I can estimate the rabbit's speed:

$$ \frac{1400 \mathrm{~ft}}{7/24 \mathrm{~sec}} \cdot \frac{1 \mathrm{~NM}}{6076 \mathrm{~ft}} \cdot \frac{3600 \mathrm{~sec}}{1 \mathrm{~hr}} = 2840 \mathrm{~kt} $$

If we think about in terms of what values are likely to be set by a human, 1400 feet in 7/24 of a second comes out to 4800 feet per second, or in other words (close to) 20 milliseconds per 100 feet—which equals 2960 kt.

Answer 2

The "flashers" part of the system is only 1400 feet long (the 15 red triangles in the diagram on the page you linked to).

If the lights blink twice per second, we can set a lower bound on the speed.

I'm assuming only one light is on at a time, and the first light in the sequence flashes as soon as the last light goes off.

Outline

• The first light flashes at time 0.00
• The last light flashes at some time no later than 0.5, 1,400 feet away
• The first light flashes again at exactly time 0.50 (making twice per second)
  Starting the the second "rabbit run".

So, the "Rabbit Run" covers 1,400 feet in 1/2 second or less.
(It could be faster, if there is some gap in time between the first and second rabbit-run in a one second interval.)

So, I think the math is: $$ \frac{1400 \mathrm{~ft}}{0.5 \mathrm{~sec}} \cdot \frac{1 \mathrm{~NM}}{6076 \mathrm{~ft}} \cdot \frac{3600 \mathrm{~sec}}{1 \mathrm{~hr}} = 1658 \mathrm{~kt} $$

To get a more precise estimate, I would want to know the time delay between the last light turning off, and the first light turning on again.

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It would also still meet the spec of twice per second if the first light flashed at time 0.00 and again at time 0.99.

That is not how I originally interpreted it, but it seems valid.

In that case, the math would lead to about: $829 \mathrm{~kt}$