Do airline pilots use speed stability to cope with unreliable airspeed indications? If not, why not?

Question

There have been a couple of airline accidents in which unreliable airspeed indications (or the fear thereof) have played a role:

• Birgenair Flight 301 in 1996, where one of the pitot tubes was blocked, causing unreliable airspeed indications. At one point, the captain gave full throttle at low airspeed and high angle of attack, causing the left engine to flame out, and producing a spin.
• Aeroperú Flight 603 in 1996, where the static ports were blocked, causing unreliable airspeed and altitude indications. The altimeter read high, and the plane crashed into the ocean.
• Air France Flight 447 in 2009, where the pitot tubes iced over, causing unreliable airspeed indications, for about one minute. The aircraft entered a stall and the pilots failed to recover from it. Presumably, the pilots were uncertain about whether or not the airspeed indications were reliable, which contributed to their failure to recover.

Now, correct me if I'm wrong, but if your airspeed indicator fails in, say, a Cessna 172, you can use the aircraft's speed stability to maintain a safe airspeed. Do something like:

1. Move the trim wheel to the takeoff position.
2. Keep the nose from rising or falling suddenly (unless you're near a stall), but allow it to rise and fall slowly.
3. If the pitch stays constant with no elevator input, you're flying at a reasonable speed. If it tends to rise, you're flying faster than that, and if it tends to fall, you're flying slower than that.

Once you get the nose to stay still, you don't have to touch the elevator much unless you change power or configuration.

Yet, I've never heard of airline pilots doing anything like this. Do they do this sometimes, and if not, is there a reason why they don't?

(For that matter, is there a reason why my suggested procedure isn't a good idea in GA aircraft either?)

Answer 1

I don't believe the procedure you've described is formally recommended. Instead there is the unreliable airspeed procedure. The exact parameters vary among aircraft types but the basic principle is that by setting a specific pitch and power combination, you're guaranteed a certain airspeed/performance.

These procedures have existed for many years but perhaps the biggest change to come from the AF447 crash is an increased emphasis on this in recurrent training.

See: What is the correct crew procedure for an unreliable airspeed alert?

Answer 2

Longitudinal stability is certainly useful part of flying with unreliable airspeed, but far from sufficient. The problem is that you can only trim for current speed, but you need to trim for cruise speed first, and then for landing speed.

There are many factors that affect the trim of the aircraft. The most important are: weight and balance, engine power and the position of the trim tabs and/or stabilizer.

The weight and balance matters a lot as evidenced by this incident. That means take-off trim is mostly useless—it is just some middle value so that the actual trim needed for take-off is not outside the authority of the elevator, but the actual deflection needed will be different from flight to flight.

So instead the unreliable airspeed procedure calls to fly level with specific pitch attitude—which results in reasonable cruise speed—and then reduce power until specific rate of descent is achieved while maintaining the pitch attitude—which results in reasonable landing speed.

Also in case of Airbus, while the angle of attack protection is lost under unreliable airspeed condition, the auto-trim function remains operational, so the aircraft still behaves as neutrally stable. This makes the procedure slightly easier, as when power is adjusted, it will keep flying straight and raise or lower nose on its own, but also slightly more risky, since there won't be pitch-down tendency before stall.

After the Air France flight 447 accident, Airbus introduced ability to show angle of attack instead of airspeed in case airspeed is unreliable, which makes the procedure much easier.

That is unreliable airspeed. Blocked static ports, as in the case of Aeroperú flight 603, are much, much worse, because they render all of airspeed, altitude and vertical speed inoperative, and vertical speed is much, much more critical. The pitch-power tables should allow setting power that will result in mostly level flight given weight and approximate altitude, but it is a lot harder to maintain safely.

Answer 3

From a basic airmanship perspective, if you are plodding along minding your own business in an airliner and all three airspeed tapes go wonky without any other changes to sound or movement, thrust, trim, etc, your basic instinct should be to do nothing and evaluate other instruments, mainly pitch and altitude, and steady as she goes. If you've been flying the same plane a long time so that the general numbers are familiar, you should be able to achieve various speeds because you can recall the pitch attitudes and thrust settings for various configurations. For example, I haven't flown an CRJ200 in quite a while but I still remember that fan RPM at Flaps 45 at Vref on a 3 degree glideslope is around 62% at MLW and around 57% at light weight and the nose attitude is about 2-3 deg down (the 200 and Challenger bizjets are weird that way). Same thing in other conditions; you know the plane well enough that you know what thrust setting and pitch gives you more or less the speed you want - good enough to keep you below Vmo and above stall at any rate. There was a CRJ crew at Atlantic Southeast Airlines in the late 00s that had a triple airspeed loss incident and they simply used really good familiarity with the type and good basic airmanship and handled it no problem.

That kind of knowledge wasn't sufficient in those accident cases as crews became mentally overwhelmed and slipped into panic mode (nobody really knows if they will or won't until it happens- good training just helps tip the probability of a good result in your favour), and in response the OEMs have all created pitch/power tables that go into the Quick Reference Handbook as part of an Unrealiable Airspeed QRH procedure. Basically, tables, derived from flight test and analysis, that say for Pitch of X degrees, N1 setting of Y, with so and so flap setting and at so and so weight range, you get a speed of more or less Z. The data is good enough to get you down safely.

On GA airplanes it's very useful to understand that the trim wheel is in fact your speed dial, like a cruise control setting on a car you might say. The speed the airplane wants to seek is based on where the trim is set, modified slightly by power setting when the engine is in front and is blowing on the tail. That is, adding power reduces trim speed on a tractor aircraft and reducing power increased it, by a few kts.

So if you are in a 172 and a wasp in the pitot kills your airspeed, as happened to me once, and you know roughly what power setting you typically use on final, and roughly what pitch attitude to use, and set the trim so it holds that, and the trim indicator is roughly where you expect it, you will be fine. If you are really too slow it should become fairly obvious as things become quiet and mushy. You aren't going to just spontaneously stall and spin if you are letting the plane do its thing at a reasonable trimmed speed. The more common problem in my experience when GA pilots lose their airspeed is they overcompensate at the other extreme and end up landing at 100 mph.

Answer 4

Capt. Reynolds suggested data charts be gathered using engine RPM, and, as you suggested, trim condition. Charles Lindbergh stressed "being as one" with your aircraft.

The better you know your machine, the less reliant the pilot is on one instrument.

In this day and age we also have GPS tracking for speed. Backups can be critically important, as is the common sense to be aware and avoid icing conditions if at all possible.

Ice buildup would not only block the pitot tubes (if heating was inadequate), but also lower the lifting ability of the wings, raising stall speed. Additionally, icing on props will reduce their efficiency, resulting in less thrust at a given power setting.

You may wish to set trim a little closer to cruise and monitor the altimeter and RPM. Then get on the radio and head for a large airport with radar tracking and a long runway. They could help with the approach.

Lacking this, a long straight stretch of superhighway or large field would do for a landing site (dry lakebeds too).