What is that thing sticking forward from the top of the tail of a B707?

Question

As an example, here's a photo: http://www.airliners.net/open.file?id=1099488&WxsIERv=Obrvat%20707-331&Wm=0&WdsYXMg=Genaf%20Jbeyq%20Nveyvarf%20%28GJN%29&QtODMg=Arjnex%20-%20Yvoregl%20Vagreangvbany%20%28RJE%20%2F%20XRJE%29&ERDLTkt=HFN%20-%20Arj%20Wrefrl&ktODMp=Yngr%201970%27f&BP=0&WNEb25u=Qnivq%20Bngrf&xsIERvdWdsY=A765GJ&MgTUQtODMgKE=&YXMgTUQtODMgKERD=1793&NEb25uZWxs=2006-08-25%2022%3A56%3A53&ODJ9dvCE=&O89Dcjdg=17679%2F88&static=yes&width=1210&height=814&sok=JURER%20%20%28qngrfgnzc%20%3E%20qngr_fho%28ABJ%28%29%2C%20VAGREINY%2048%20UBHE%29%29%20%20beqre%20ol%20ivrjf%20qrfp&photo_nr=21&prev_id=1099516&next_id=1099452&size=L

Answer 1

A high-frequency antenna mounted on the top vertical stabilizer.

Aloha

Answer 2

OK slow down everyone. It is definitely not at pitot probe, those are on the sides of the flight deck. It has nothing to do with the aerodynamics.

It is absolutely the High Frequency (HF) antennae. I know this because I am an E-3 flight engineer, and the E-3 is a modified 707. We have the exact same antennae on our aircraft. Although on some aircraft there is a wire that extends from the tail to the body, this is not he case in the 707. HF radios are used for long range communications. A commercial airliner would use this for maintaining contact with ATC outside of VHF radio range.

Answer 3

Well, it's definitely not a pitot tube; a pitot line running down the vertical stabilizer and all the way forward to the electronics bay below the cockpit would not be a good idea; you'd end up with a huge potential for leaks. The pitot tubes are up front, below the cockpit.

It's an HF antenna, used for long-range communications. Since the 707 was designed for flying trans- and inter-continental routes, it needed some means of maintaining communications beyond what the standard aviation band VHF radios can, which are line of sight (like TV and FM radio signals).

This type of antenna was state-of-the-art in the 50's, when the 707 was designed. Since then, advances in antenna technology have made it possible for aircraft manufacturers to use much smaller and more aerodynamic HF antennas. This type of antenna was at least an improvement on the older 'long wire' HF antennas, which had to be reeled out behind the airplane in flight.

The HF 'stinger' antenna makes it very easy to identify a 707 on a ramp full of airplanes; it's the only one that used this part of the airplane to mount an HF antenna of this type. Some military versions add additional HF 'stinger' antennas on one or both wingtips as well.

There is a sort-of pitot tube on the 707 vertical stabilizer, about a quarter of the way up from the fuselage. This one is known as the 'q-tube' (at least in the civilian world). This takes dynamic 'ram' air pressure through a duct to a 'q-bellows' inside the tail, which makes small adjustments to the flight controls. The q-tube is fairly small, maybe a foot long and about 2"-3" in diameter, so it's easy to miss. Later Boeing airplanes mount a standard pitot tube on both sides of the vertical stabilizer, for the same reason.

The first link below is to a picture of the tail of John Travolta's 707, which clearly shows both the HF 'stinger' and the q-tube. The second one is of an Austrailian Air Force 707, which shows two pitot tubes, covered with a protective sleeve with 'Remove Before Flight' streamers attached, with a bungee cord attaching them to the airplane. (That smaller thing sticking out of the airplane behind the pitot tubes is a TAT probe, for 'Total Air Temperature'; it's basically a thermometer that feeds data into the air data computer.)

Later . . .

Way back in the early days of aviation radios, HF antennas were reeled out of the airplane. Federal regulations in place back in the 30's and early 40's required airliners travelling overseas to be able to communicate on 500 kHz, which was the international maritime distress frequency. The radios and antennas of the day were only able to reach that low by reeling out a long wire antenna (albeit one much shorter than the truly long wire antennas used for transmitting on LF and ELF frequencies today by the E-6 TACAMO airplanes, for example). Anyway, all this is getting a little off-topic, which was 'what is that thing sticking forward from the top of the tail on a B707?'

Answer 4

Let me add some to the flight engineers information, he is correct it is an HF radio probe antenna. The wire antenna for HF was never reeled out as one post says, that is a Low Frequency (LF) or Very Low Frequency antenna, and still in use on some military aircraft. The wire antenna and probe antenna for HF (2.0000 - 29.9999 MHz) is used when traveling over the Atlantic and Pacific oceans. The aircraft makes position reports to the International Civil Aviation Organization (ICAO) for the purpose of flight following while they are not under RADAR control.

Answer 5

Its called a Pitot probe. Is usually used to measure air speed and such. Or is used for whatever the manufacturer designed it for.

Answer 6

That's an aerodynamic strut designed to hold the end of a wire radio antenna.