Cues to use when indicated airspeed goes missing
In the summer of 1964, Cadet Detwiler was home on leave from the U.S. Air Force Academy and flying around Northwest New Jersey with friend Marv Everett, whom everyone called “Sonny.” Although I had not yet had any military flight training, I had just earned my Private Pilot license and was feeling pretty competent with nearly eighty hours logged and having taken a course in acrobatics in the academy's two venerable T-34s.
Sonny's father was the chief707 instructor at Pan Am at that time and had granted us use of his 172 for the afternoon. Sonny had been well taught by his dad and had a few tricks up his sleeve I had not been exposed to in previous training. About half way to Solberg on a leg from Blairstown, Sonny reached into a side compartment of the 172 and placed a round rubber suction disc over the airspeed, completely covering that instrument.
“Just fly the airplane,” says young Mr. Everett.
I had to admit to a little trepidation at the prospect, but also figured Sonny wasn't going to let me bust up the old man's machine. We began the let down into Solberg and, to my surprise, easily completed the approach and landing on a partially paved Runway 22.
“Airspeed is vital, but you don't have to see it to have it,” said Sonny. “You just have to make sure you have enough of it by whatever means possible. In this case the look and feel of the airplane provided those cues.”
Fast forward about five years. I was stationed at Craig Air Force Base in Selma, Ala., and looking for something interesting to do in the time off between sessions of “teaching little fingers to fly the T-37.” I gravitated to a little grass strip on the south side of town where Jewell Flying Service maintained a fleet of Piper Pawnees and Grumman AgCats for aerial application. I talked to the owner, Mr. Jake Jewell about working for him in my off time.
“The only way I'll let you fly for me is if you get your own airplane. I'll take care of it for you, but it has to be your airplane. “ Turns out Jake had let another young man talk him into flying one of his airplanes, and that fellow had been killed in a Super Cub when he stalled on an end-of-field reversal. Jake carried that thought with him and probably does to this day.
I bought a Stearman.
Although I never did become fully qualified by Jake's standards, the Stearman provided over 100 hr. of fun and excitement. The only instrument that worked on the airplane was the oil pressure gauge. The pitot tube was so full of goo that there was no airspeed above an occasional flicker on the needle. The altimeter would stay stationary and from time to time jump a few hundred feet. It was seat of the pants flying and I loved it.
The Stearman carried so little payload that eventually Jake let me fly the small, 450-hp “Cat”. We'd land those planes on country roads and in farmers' fields to reload. By the time I finished working with Jake, I had an excellent “feel” for flying light airplanes without airspeed indications.
Let's carry this discussion of flying without airspeed into today's headlines. I dislike talking about pilots who have died in airplanes, because we can never know everything that went wrong. It is possible to blame something on pilot error without knowing that the pilot may have handled everything in the situation into which he was thrust without being able to maintain or recover to controlled flight.
Nevertheless, accidents allow us to put ourselves in the position of the pilots involved and at least ask, “What might I have done in that situation?” That is one reason why I read Dick Aaron's Cause and Circumstance column so carefully each month. We have to talk about, discuss, argue and study the misfortunes of our fellow pilots to learn from those situations. Not to do so would be foolhardy.
Two winter accidents point to icing as a causal factor. Icing can take away airspeed indications. Did the Dec. 20 crash of a Socata TBM 750 in northern New Jersey have a root cause in the same occurrence that brought an ATR72 out of the sky over Roselawn, Ind., in October 1994? Did the TBM pilot have the plane on autopilot, let it ice up, and then not realize that the autopilot, in an effort to maintain steady flight, had led him down a fatal path? If his airspeed had iced up, was he even unaware that it was dangerously slow? Was the airplane so heavy with ice that “normal” airspeed was not enough? These are questions we should all ask and answer regardless of what actually happened in that incident. We owe it to our passengers
Let's move the discussion to icing and lack of airspeed indications in jets. Flying a jet without the benefit of airspeed indication is a different proposition altogether. High speed Mach buffet feels very similar to stall buffet. In fact, I have talked to people who wound up at unbelievably high speeds when their pitot static systems iced up and they misinterpreted the buffet for stall. Regardless of the fact that the dive shouldn't have happened, fortunately for one person with whom I spoke, he broke out of the clouds and recovered visually.
Add to this the modern engineering concept that “we build airplanes that cannot stall,” and the stage is set for some dramatic results if the capabilities and the limitations of modern fly-by-wire (FBW) systems and cockpit displays are not fully understood and used by the aircrew piloting those airplanes.
I think the crash of447 will provide the professional flying industry a source of discussion and concern for years. It will become what Eastern 66 or Delta 191 were to microburst and wind shear events. Purported cockpit conversations and actions taken by crew members, whether official or not, provide a copious source for discussion of such topics as cockpit resource management, thunderstorm preflight study, FBW systems, modern attitude instrumentation, and aircraft icing.
To review, it appears theflew through the tops of some strong convective activity shortly after its departure from Rio de Janeiro on the evening of May 31, 2009. The flight was too heavy to climb higher at that time and the pilots found themselves in the lower 30's during the weather penetration. The aircraft pitot static systems iced up and the pilot flying wound up with a handful of heavy machinery at a high altitude when the autopilot clicked off. As I've previously written, the pilot probably saw his indicated airspeed increasing and he vigorously pulled back on the stick, zooming the airplane thousands of feet and nearly resulting in a high altitude stall. The FBW system prevented the stall, but held the airplane in a nose high “flying” attitude just below the stall angle of attack. This initial “uncontrolled airspeed” flight appears to have panicked the younger pilot and set the stage for disaster.
For some unknown reason, in the transcript I read, the senior pilot's only response to the zoom was to warn the other pilot to “ease off a little.” He should have said, “I've got it” and told the other pilot to sit on his hands. It appears, from what I've read, that the junior pilot's continued reversions to full aft stick on the right, without the apparent knowledge of the more senior pilot, caused the fatal high angle of attack and resulting sink rate to be resumed and held until the airliner crashed into the Atlantic.
Although the captain had retired to the rear for his scheduled rest period, the more senior copilot, sitting in the left seat was well experienced. Had he taken, and maintained, control of the airplane, there can be little doubt from what I've read that the accident would not have happened. But did he fully understand what was going on at the time? Had the frozen pitot static system also confused him? When they did pop out of the top of the clouds, the airplane's de-icing systems “thawed” the pitot static systems. They had normal indications of airspeed and altitude from that point. Did this lead them down a path of reasoning that said that the full aft stick and zoom had saved the high airspeed situation?
Sonny Everett's observation of years ago: “Airspeed is vital, but you don't have to see it to have it. You just have to make sure you have enough of it by whatever means possible” holds true today, and will forever. If the triggering event in the Air France accident was the loss of airspeed and altitude indications, what actions could have been taken to quickly and safely stabilize the airplane? As noted in “Dome of Confusion,” (October 2011) all the pilots had to do was put the attitude of the airplane a degree or two nose low with climb power for a few seconds and then resort to level flight attitude and maintain. After further discussions with other aviation professionals, including Bob Hare, an instructor at CAE's Morristown, N.J., learning center, it turns out the solution is even easier.
Modern attitude indicators are far more information intensive than older ones, and make “airspeedless” flying a lot easier. In Figure 1 the attitude indicator is just like the one in the 7X and similar to many modern instruments. In addition to attitude of the airplane, as depicted by the yellow aircraft wings, there is a little winged green circle that indicates aircraft's flight path. On the left side of the instrument is a green “carrot” that indicates acceleration. Ignore, for this discussion, the purple indications as they represent flight and thrust directors. On the airplane, they could be cleared while we tried to maintain basic control. In effect we have, on the attitude indicator, airspeed cues, altitude cues, as well as attitude.
Let's break it down.
When we put the green flight path indicator on the horizon, the airplane is neither climbing nor descending. This little cue is one reason that the steep turns that are still part of the PIC check have become so easy. You do not have to constantly “update” an attitude to maintain level flight. Just keep the flight path indicator on the horizon and altitude is nailed.
OK. So the flight path is constant and we are headed neither up nor down. How do we know that the power setting we have will be enough to maintain that level flight? What if the airspeed continues to deteriorate and eventually we cannot hold the airplane on the level flight path indication?
Look again at the attitude indicator. Flight path is the opposite of relative wind, by definition, and “where the wings are pointed” is another way of saying “chord line,” at least for this exercise. The angle between chord line and the relative wind is defined as angle of attack.
If we bring the airplane to level flight via the flight path indicator, but the angle between the flight path indicator and the airplane attitude is increasing, the aircraft is slowing. But if the angle between the two is decreasing, the aircraft is accelerating. It's as simple as that. This can be confirmed by observation of the acceleration cue. The object then is to keep the flight path indicator on the horizon and keep a stable angle of attack through use of the acceleration cue.
Let's take this exercise a step further. Obviously maintaining safe airspeed is the goal, but why not shoot for the correct airspeed. Flying the “correct” airspeed in the North Atlantic Track System, even if we've turned off the tracks, could be considered the “safe” airspeed.
Say you have a flight plan that you have been checking all along for accuracy. That plan will have a predicted ground speed for each area of flight. Remember that ground speed is one of the first iterations performed by the inertial reference systems. It's not subject to icing problems. If you know the accuracy of the flight plan to that point, you can use the acceleration carrot to add or subtract thrust until getting near the planned ground speed, which in the 7X, for example, is read on the lower half of each PDU. At that point, bring the acceleration carrot to the wing line, maintain the flight path on the horizon and maintain the angle of attack, thus the groundspeed. Of course the groundspeed will have to be updated with the flight plan if you don't “thaw out” in a relatively short time.
During recent online discussions about AF447, there have been a lot of reflections that the FBW system inputs, or ignorance of those inputs by the pilot not making them, may have had an impact on the flight's outcome. However, in the 7X, if I had taken control of the airplane and the other pilot touched the sidestick, it would vibrate vigorously.
The only unknown here is what the FBW systems would do if they “thought” the airplane was accelerating. These systems are designed to protect the airframe from overstress and from stall. We were taught that they use airspeed inputs to make those determinations. If true, and the airspeed indications are false, the pilot has to have the ability to put the FBW system into “direct” laws, which means it responds directly to pilot input. In that circumstance, the computers have no say and the pilot can put the attitude wherever he wants.
With direct laws, plus the preceding explanation, you're “flying without airspeed.” BCA