<p>On an active US Navy S2 Tracker Anti-Submarine-Warfare plane, this was where the boom that held the Magnetic Anomaly Detector retracted. Those TLAs are ASW and MAD, respectively. The MAD boom was retracted when the plane was on deck or on land, and extended in the air when searching for submarines. The theory, naturally, is that a large, steel, submarine hull will create distortions in the Earth's magnetic field, whether or not the hull is "demagnetized". The detector had to be pretty sensitive to read a steel hull that might be 1000 feet (300m) or more below the airplane, so hanging it out the back of the airplane on a fibreglass pole made a lot of sense.
When this S2 was rebuilt as an air-tanker to drop fire retardant on wildfires, the MAD boom was removed and replaced with a big fire-hose, leading from this relatively safe spot well away from the propellers to the retardant at the bottom of the fuselage, below the wings. The pilot could land, taxi the plane to a refill station and one person on the ground could refill the retardant tank in a few minutes, without the pilot even shutting the engines off (hence, "hot")
</p><p>On this example, which suffered a belly landing or some other incident that bent its propellers, the retardant tank was removed and with it the hose to refill it. So we just see a tunnel going forward into the fuselage. But once it was more...
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<a rel="nofollow" class="external text" href="https://www.flickr.com/people/9998127@N06">Bill Abbott</a>
<p><a rel="nofollow" class="external text" href="https://www.flickr.com/photos/wbaiv/4311289530/">CDF (piston engined) S2 Tracker fire retardant re-fill point</a>
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