Wednesday, March 23, 2016

What Makes a Go-Around So Dangerous?



Distractions and fatigue make tough go-arounds even tougher
A FlyDubai 737-800


Even though the investigation into the crash of FlyDubai 981 is still in its preliminary stages, one of the more plausible accident scenarios being discussed is that the aircraft attempted a go-around from an approach and subsequently stalled causing the high descent rate and impact on the runway. Video which captured the accident shows the aircraft in an extremely steep descent which is indicative of a stalled condition. And in fact the last transmission from the aircraft to the tower was that it was going around. So why are go-arounds so dangerous?

A go-around is simply a maneuver whose purpose is implied by its name. If at any time during an approach the pilot feels that for whatever reason that landing isn't the best idea, he or she aborts the landing and "goes around" for another try. This involves adding power and climbing away from the runway for another shot while retracting the landing gear and flaps or "cleaning up" the aircraft.

Sounds simple, right? Well for various reasons go-arounds seem to be one of the most stress inducing maneuvers for pilots. It is also one of the most critiqued items during checkrides as a result of being performed incorrectly. But to be perfectly truthful, a search of the NTSB database turned up only three fatal accidents attributed directly to go-arounds since 1985, at least in airline, or Part 121 flying. In general aviation, however, the number is much higher at 273 fatal accidents since 1982 attributed to go-arounds.

Those accident results suggest that pilot experience may play a factor in the successful completion of go-arounds as many general aviation pilots don't have the experience that airline pilots do. Still the go-around can be deceptively difficult and due to the low altitude nature of the maneuver leaves very little room for error if performed incorrectly.

And while the NTSB results do not capture low-speed events which did not result in a stall or accident, those types of incidents are now being identified by new data recording and analysis equipment recently installed on most airliners. These data suggest that low speed and approach-to-stall events are more prevalent than previously thought.

So What is it About Go-Arounds that Makes Them Difficult?


There are two aspects to a go-around that can make them difficult to fly and costly to screw up. The first is that the maneuver is performed close to ground. Any mistake made at low altitude has less time to be corrected before terra firma ends the flight in an abrupt fashion. The second aspect of the maneuver is that the entire vertical vector of the aircraft has to change. 

By this I mean that your 30 ton airplane which is descending at perhaps a vertical velocity of 800 feet per minute while on approach has to have its downward momentum stopped, reversed and flung back into the air at perhaps 1000 to 2000 feet per minute. It is this reversal of momentum which causes the most problems. The problem is one of "energy management".

Energy management is best illustrated by thinking about an old style roller coaster and is central to flying an aircraft. As a roller coaster tops a hill, its speed is slow but the potential energy is high. As it hits the trough, the potential energy is low but the speed, or kinetic energy, is high. The same principle applies to an aircraft as pilots can often trade airspeed for altitude or vice versa.

During an approach, however, both the speed and altitude are necessarily low. The aircraft is flying at only 1.3 times its stall speed and has both gear and flaps extended which add drag. It has a low energy state. The only way to get the airplane back up in the air from this position is to add a lot of power from the engines. But this addition of power must be accompanied by a coordinated and precise pitch adjustment.

Pitch control during a go-around is extremely important. The use of insufficient pitch, or keeping the nose too low, may cause the aircraft to merely accelerate but not climb. Use too much pitch, or raise the nose too high, and the aircraft will climb, but the speed may drop. Remember that at this point the aircraft is already very close to stall speed. Stall the aircraft, and it drops like a rock with little chance for recovery at low altitude.

One last characteristic of most airliners is that their pod-mounted engines are hung below the wing. What this means is that an addition of power causes the nose to tend to come up as the thrust vector is being applied from below the wing. This tendency must be anticipated and countered to prevent the pitch from getting too high.

Distractions and Fatigue Don't Help


So we've decided to go around, put the power in and are climbing away from the ground. What happens next? It gets extremely busy is what happens next. The tower will be barking instructions at you with headings and altitudes to fly or you may be scrambling to read your previously issued climbout instructions and approach plate. You will also have to clean up the aircraft without overspeeding anything. This means retracting the gear and then the flaps on their speed schedule. You'll also be getting a frequency change right about now to return from tower to approach control. Not blowing through your assigned altitude is also somewhere on your plate since you are now climbing at full power.

There's a hierarchy of flying priorities that many a flight instructor attempts to instill in their students: Aviate, Navigate and Communicate (and in that order). Forgetting to do these tasks in the proper order can result in a bad outcome. Or as an old aviation commandment instructs: 

Thou must maintaineth thy airspeed lest the ground reach up and smite thee.

A mention of fatigue is in order here. All of these things are challenging to do on any given day, but being fatigued, as has been alleged of the FlyDubai pilots, makes them especially tough. My personal experience with fatigue is that while you might feel alert during a tricky approach and go-around, channelizing attention on one particular item is very easy to do when fatigued. This means that instead of simultaneously managing many different tasks in the cockpit during a go-around, it is easy to become fixated on one particular item at the expense of others

The danger of course is that of fixating on say a course change while dropping your pitch and airspeed out of your scan. This one mistake, if not corrected quickly, can doom an airplane. Recovering from a stalled condition takes many thousands of feet of altitude if the stall is severe enough, and that is altitude you just don't have.

Go-arounds, if you happen to be on an airplane when one happens, are really no big deal and I don't mean to make any nervous flyers more so. Many times they are for mundane things like spacing too close to the previous airplane or being directed by the tower. I personally like flying them as they're a challenge and something a little different from the routine. Just know that when they do happen, the guys or gals up front are really busy.









4 comments:

  1. Absolutely brilliant and informative piece - thanks very much indeed. I have been a passenger in a number of go-arounds - I will remember to thank the aircrew properly next time. As a physician on call for many years I can identify strongly with your description of fixating on one function when fatigued - its a very real risk and an important reason why we work in teams.

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  2. There's little doubt that teams working together are one of the best defenses against fatigue. The insidious nature of fatigue is what makes it so dangerous.

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  3. Appears some are pointing fingers toward somatogravic illusion as being a major contributor. While I see this in GA (i.e. JFK junior) don't recall any recent air carrier accidents attributing it to cause. What's your thoughts, Rob?

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  4. Hi Mike,

    That could easily be the case. A light airplane on a full power go around can easily be disorienting. Throw in fatigue and IMC and it's a bad mix. I also wonder if their autopilot disconnected. Some Boeing autopilots will and some won't.

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Capt Rob