The Pitot tube is ubiquitous on aircraft because it offers a cheap, reliable and accurate way to measure airspeed. But the device is also vulnerable to icing, which is why a group from the University of New South Wales wants now wants to supplement the airspeed indicator with lasers. Using the low-cost Doppler techniques found …
This idea resurfaces...
every five years since at least the late 70s -- since there's an aviation disaster related to anomalies in the pitot-static system about twice a decade. I was involved in one such system in the mid-90s that eventually turned into a fully qualified system from BAE. But every time, the combination of pressure from airplane manufacturers and inertia at government regulators means that the technology gets mothballed yet again. Unfortunately, memories of AF 447 have already faded, so I suspect that laser doppler velocimetry will be shelved for yet another half-decade until the next pitot-related aviation disaster.
Re: This idea resurfaces...
I can understand how keeping the laser in the body where it's easily de-iced helps one aspect of the measurement, but what about at the reflector and everywhere in between? What if it's the tube that's iced and therefore unable to reflect cleanly? Or what if the aircraft is flying through a cloud that introduces ice crystals or other particulate matter into the beam's path? How would their system account for opaque particulate interference?
Re: This idea resurfaces...
> What if it's the tube that's iced and therefore unable to reflect cleanly?
I've flown on many research flights and seen icing on probes. Ice builds up on the leading edge of the probe, but not on the sides - I assume that any ice/water besides the probe is swept away in the airflow. The end result is a stick of ice at the front of the probe no wider than the probe itself. They can grow quite long.
> what if the aircraft is flying through a cloud that introduces ice crystals or other particulate matter into the beam's path?
If this is truly a Doppler-based system then the sideways motion of any interrupting particles shouldn't affect it - Doppler detection measures the along-beam motion. If there are enough particles to actually block the line-of-sight then you've flown into an iceberg.
TAS / IAS
For the most part pitot / static systems show Indiacted Airspeed yet still account for True Airspeed. Some clever gubbins will be required to calculate IAS with this.
Also angle of attack variation may be different too.
Sure, blame the pitot tube.
Air France flight 447 crashed because the co-pilot stupidly pulled *back* on the stick while the stall alarm was sounding. He kept pulling back all the way down to the sea - more than three minutes of stalled falling. Rookie error: one of the very first things any pilot at any level is taught is how to prevent a stall, and pushing *forward* on the stick is the best way to do that.
correct me if I'm wrong
but the artist's impression is of a laser shining on a..... Pitot tube.
Re: correct me if I'm wrong
You're quite correct, the caption from another version of this story is "Artist's impression of the laser-based sensor augmenting a Pitot tube. Credit: Sven Wittig".
Pitot Tubes are Already Heated
As a pilot I'm all for any reasonably-priced backup system that can prevent accidents, but pitot tubes are already heated to prevent icing even in light GA aircraft. While the pitot-static system can fail for any number of reasons including icing/contamination of the pitot tube or static port, clogging of the lines or instrument failure, pilots are trained to recognize these failures and fly using the remaining instruments. This is called flying "partial panel" and is a huge part of instrument training. And I agree with "Anonymous Coward"... the Air France crash was caused by pilot error. They had a working stall warning and ignored it.
Of course, if this device is cheap and reliable then why not use it?
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