# What were the advantages of the L-1011 AoA sensor design?

The Lockheed L-1011 had an unusual type of AoA sensor. Instead of the usual vanes it used a tube with two rows of holes in it about 90° apart. A diaphragm sensed the pressure differential between the two sets of holes and a motor adjusted the sensor until the differential went away. The AoA was them determined from the angle the sensor was to the fuselage.

Source: NTSB accident report TWA 843

This seems particularly complex, requiring a motor and possibly introducing lag into the reading. Why did Lockheed decide on such an unusual sensor design? Does it have some a advantages to a conventional vane? Was it used on any other aircraft besides the L-1011?

• You can make your own AOA sensor for your homebuilt airplane that way, using a fixed horizontal tube with the two rows of holes in an upper and lower chamber, and connecting each chamber to the pitot and static ports of an old airspeed indicator with the face redone with AOA markings. You had to calibrate it yourself though. – John K Mar 14 at 18:34
• Hypothesising: Maybe one advantage is that the moving parts, mechanics and electrical sensors are somewhat protected, given they’re inside the tube or even inside the fuselage? – Cpt Reynolds Mar 14 at 21:20
• Speculation: it's easy to detect faults. If there's a pressure offset even after you applied motor commands, then you know the tube is jammed. Likewise, instead of servoing for zero pressure difference, you servo to track a sine wave offset. If the tube is blocked, you'll quickly see the tracking error. – user71659 Mar 14 at 22:51
• The X-15's nose mounted Q-ball alpha/beta sensor worked the same way (a servo system rotated the ball until the pressures were equal, and then the angle of the ball was read). It was hydraulically powered! Good diagram on p. 5 nasa.gov/centers/dryden/pdf/87711main_H-374.pdf – Organic Marble Mar 16 at 14:25

I found several patents that resemble the L-1011's, dating back to at least the 50s. One such patent remarks that there are two main types, the vane, and the rotating pneumatic tube (also comes in conical shape).

The advantages of the latter when compared to the vane-type are:

[B]etter balancing characteristics, lower air friction, greater sensitivity, and less tendency to flutter or otherwise yield erroneous readings under adverse flying conditions. Also, it requires less power to prevent ice accumulations on a probe than a vane.

A disadvantage is keeping the moisture/dirt out:$$^1$$

There is one difficulty in making this a rugged instrument. The movable cylinder must be mounted to move as freely as possible so as to have good sensitivity. This requirement makes it difficult to make a good seal around the moving cylinder to keep out dust and moisture.

A motor would solve this seal issue. But the advantages of the form should hold whether motorized or not. A more recent patent uses the tube design to combine the AoA and static/dynamic pressure sensors into one probe.

Was it used on any other aircraft besides the L-1011?

The F-14, for example, uses the pneumatic (conical) type:$$^2$$

(primeportal.net) The left (port) side of the F-14's nose.

$$^1$$ https://apps.dtic.mil/dtic/tr/fulltext/u2/102442.pdf page 50.
$$^2$$ The location is confirmed by the F-14D manual (only 1 probe), and it's referred to as probe (not vane).

• -1 Hand-drawn red circle is neither hand-drawn nor red!!1! – David Richerby Mar 15 at 10:35
• @DavidRicherby: fixed – oh wait I didn't use Paint, brb – done, the line has to be 1 pixel wide :P – ymb1 Mar 15 at 10:56