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Aviation Safety

AVIATION SAFETY

Dave Fry, WVFC CFI and Aviation Safety Counselor dgfry@aol.com

Here There Be Dragons

What kid didn’t love the old maps of the “New World”?  Of course, most (as we know today) were horribly inaccurate, but that didn’t keep it from being exciting to think about those ancient explorers, and about their skill with primitive equipment and their bravery heading into the unknown.

Some of the old maps even had the notation, “Here there be dragons.”  WAY too cool.

There are a variety of dragons in aviation, any one of which will bite you in the empennage, and in this column I’m going to describe a few of them, most of which come to mind as a result of the Asiana 777 crash at SFO a few days ago.

As of this writing, the cause of the accident is still under investigation, and there has been little except speculation, which is not the object of this column.  Granting that my jet experience is in planes with gross weights on the same order as the reserve fuel on the 777, there are some factors, rules, and procedures that apply to both.  So mostly what I want to do is describe how things are supposed to work, and where the dragons are – those things that will eat you alive if you don’t do them right.  I’m not suggesting that any of them applies directly to the Asiana crash, but they are the first things that come to my mind.

The Stabilized Approach:

Shortly before the descent is started (or immediately thereafter if ATC trumps your plans and starts bringing you down before you really want to descend), the PNF (Pilot Not Flying) fills out the Landing Data card, which among other things includes the Vref airspeed.  This is the target airspeed that will be flown (in the approach configuration) continually from the Final Approach Fix inbound.

The PNF listens to the ATIS, AWOS, or ASOS and modifies the computed Vref to account for gusts and so on, then briefs the PF on both the weather and the computed and modified Vref numbers.  After the PF agrees, both pilots set the speed bugs on their airspeed indicators.  Then either the PF or the PNF briefs the approach with the other confirming frequencies, headings, courses, and altitudes, as well as the missed approach procedure.

Outside the Final Approach Fix, as the plane approaches the glide path (ILS, VASI, LPV, or PAPI), the PF reduces airspeed and calls for the appropriate flap and gear settings, so that the plane is between Vref and Vref +10 (but we’d rather be at Vref+5 KIAS), and descending on the desired path.

Regardless of the approach type, the PNF approximately every 15 seconds gives a call like, “On glide path, on localizer, ref plus 5, 1000 to minimums.”  In between these call outs, the PNF also gets to read the checklist (challenge and response for some items, simply doing and reporting to the PF for others), handle all radio calls, and watch outside for signs of the runway.

Precision approaches have a glide path that would result in hitting the runway at the 1000 foot mark if the pilot does nothing but follow the glide slope to the ground.  So, at 50 feet above the runway (which usually occurs right at the threshold), the pilot brings the throttles to idle and keeps flying the same path.  This results in an increased pitch as the airspeed drops and the plane enters ground effect.  The sink rate decreases as the plane slows and approaches the runway until at the 1000 foot mark and exactly on the centerline, the plane lands smoothly and rolls onto the runway.  At this point, the pilot gets really busy, pushing the nose down (a bit counter-intuitive to the average “hold the nose up on the follow through” 172 pilot), activating the spoilers and speed brakes, deploying the thrust reversers, and applying reverse thrust.  The PNF calls out two speeds, the one at which the PF may begin braking, and the one at which the PF must stop using reverse thrust.  Both sides of the cockpit tend to be pretty busy in the last 50 feet of descent and the part of the landing that happens before taxiing off the runway.

The Penalty (Part 1) – Self Imposed:

If the plane exceeds Vref +10, drops below Vref, exceeds 1000 fpm on the descent, drifts more than ¼ scale off either the vertical or horizontal guidance, or exceeds 30 degrees of bank (on a circling approach), it is an automatic, MANDATORY go around.  No questions, no arguing, just cram the throttles and GO!

Now, this is expensive in terms of fuel (turbines are REALLY thirsty at low altitudes), additional time on engines, each of which has an operating cost measured in thousands of dollars per hour, to say nothing of the loss of passenger confidence in your piloting ability.  As a result pilots try to avoid go-arounds, but they do it by staying within the rules, rather than by trying to “save” the landing.

The Penalty (Part 2) – Where the Dragons Are:

Despite the many really cool features of jets (big ones in particular), there are a couple of things that are less than completely positive, and in fact, are part of the reason for doing things the way they should be done.  Three things come immediately to mind: Weight, Response, and Complexity.

Big jets are heavy, and when they get moving in a particular direction, they tend to keep moving in that direction.  Newton had something to say about that, and in the case of big jets, they put the “mo’ ” in “momentum”.  As a result, it is often worse to come in high (resulting in a steeper approach and a higher sink rate) than it is to come in low – there is a spare engine (or so) to keep the low approach from plowing the ground leading up to the runway if an engine quits.  High sink rates mean the plane needs to generate a LOT of lift to stop the downward momentum, and at Vref the plane is at or near the part of the power curve in which an increase of angle of attack results in LESS lift rather than more.  So the attempt to stop a steeper than normal descent can easily result in less lift rather than more.

Worse, when descending from above glide path, the pilot has two equally bad choices: (1) Speed up to descend faster, and (2) reduce the power even more.  A pretty good rule of thumb is that an extra 10 knots on final is equal to an additional 1000 feet of runway required for the landing.  The alternative of reducing power is equally unattractive.  When the power is at idle in a big jet engine, the pilot can cram the throttles, get up, leave the cockpit, get some coffee, chat with the flight attendant and return to the cockpit to find the engines still in the process of spooling up.  OK, so it’s not QUITE that bad.  But you’re still going to be getting closer to the ground for a long time while awaiting the power increase.  The delay can easily exceed 7 or 8 seconds, and any pilot will know that on final approach, 7 or 8 seconds is only a rock throw from eternity (one way or another).

Because engine power doesn’t change instantly (big planes, despite the light control feel on many of them, are much more like Fruehaufs than Ferraris), speed control is critical, and that means someone (PNF) needs to be watching it constantly, and calling it out to the PF.  Worse, because the plane is heavy, even when the power has been changed, it may be a while before the speed actually changes.  Meanwhile, the plane is getting slower and closer to both the ground and the stall speed.

Finally, despite the automation, or perhaps because of it, there is a lot going on in the cockpit of a big plane, especially as one approaches the runway.   This is one of the reasons all larger jets are certified for two-person crews.  Editorial comment:  Smaller jets should be flown two-person crew, as well.  I’m certified single pilot in Citation Mustangs, and have flown them single pilot occasionally, but I know I’m a FAR better pilot when I have someone else in the cockpit with me working in a CRM environment.  But having another person in the cockpit isn’t enough; it has to be done as a crew, with well understood roles, responsibilities, and communication protocols.  When CRM breaks down, any number of bad things can (and do) happen.

Conclusion

There are a variety of dragons that await any pilot that doesn’t carefully follow the required procedures (this, BTW, is one reason insurance companies have such incredibly strict experience, training and currency requirements for jet pilots).  It will be interesting to see if one (or more) of these dragons bit the Asiana crew.

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