There’s a concept called ‘tolerance stack-up’, an example of which can be found in engine building. Wherever moving surfaces meet, a supply of oil is essential to minimize wear and restrain heat buildup. When assembling an engine, care must be taken to measure and record the gaps (tolerances) between each of these moving surfaces. An extra thousandth of an inch might seem negligible. But, if an engine has excess tolerance in each of a dozen or more bearings, the result is extra gap between those surfaces that oil must fill. An engine oil pump will produce flow that increases as engine RPM increases up to a point beyond which further increase of RPM will not produce more flow. If the tolerance stack-up is too great, oil pressure will not build, and the loose engine will experience higher temperatures and an increase in wear.
Mistakes that we make in all phases of flight can “stack-up” too, and the result can be fatal. The decision to write this was a difficult one. Because I’m a low time private pilot, (425 hours, high performance and complex endorsements) I am not in a position to offer advice to anyone. I consider my ticket to be an earned opportunity to learn. But perhaps another beginner pilot will take something useful from my experience, so my pride notwithstanding, here is what happened.
I had a planned flight with a passenger scheduled the following Monday, and as it had been a while since I had flown a light, high wing airplane, I figured a refresher would be a good idea. It was a usual Midwest summer Saturday. Nothing in the METAR or TAF for the Spirit of St. Louis Airport, KSUS (hint – mistake 1) provided any reason for concern about my planned local flight.
I scheduled the last remaining aircraft – the lone and very used 172R model in the fleet. (hint – mistake 2). I had flown that aircraft in the past and recalled thinking that there would not be a second date. (hint – mistake 3).
The preflight was normal. I set up my Stratus and Foreflight, requested a departure heading, and was soon airborne and headed out of the Class D airspace and toward Washington Regional Airport, KFYG, which is where students from our flight school typically carved up the pattern and learned just how much spring there was in the Skyhawk landing gear.
I sequenced into the pattern and made three decent landings. The pattern was too full of airplanes for my taste, and I announced a downwind departure. As I climbed out and to the south, I thought I noticed a change in the winds. I tuned in the ATIS frequency for KSUS and was unpleasantly surprised to hear that the wind was now from 210 degrees and gusting to 17 knots. I calculated that I would have a left crosswind between six and 13 knots. I called tower and reported inbound. I asked about the wind and was advised that the METAR was 2108G17KT, but he was seeing wind variable from 180 to 210 degrees. I continued inbound for runway 26R. (hint – mistake 4).
26R is right traffic, but tower gave me a left downwind for traffic and he would call my turn. As I flew along, I was certain he had forgotten me, so I made a call whereupon he immediately called my turn and cleared me to land. I adjusted my thinking for a straight in approach. I had learned to fly using 26R and I knew the pattern, the landmarks that worked for me, and the sight picture. I picked out the U-shaped building that I always used to initiate my base turn and determined that I would cross that at 70 knots. The ride was smooth and gust free. I crossed the landmark building at 70 kts. and had two red and two white on the PAPI. My approach was stable. (hint – mistake 5). It bears noting that there is a 400′ bluff just south of the field at KSUS. When winds are from the south, that bluff becomes a salient factor for pilots.
I always read the accident briefs published by Plane and Pilot Magazine. In each, I try to find a lesson that one day might save my life. It never occurred to me that I would be the subject of one of those accident briefs.
Probable Cause and Findings
The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The pilot’s failure to maintain control of the airplane while landing in gusting wind conditions, which resulted in a hard, bounced landing and the nose landing gear collapsing.
The Stack -Up
My cursory review of the METAR and TAF should have been augmented by a review of the various other resources available. What does the TAF indicate at other airports in the direction from which the wind was emanating? An app called Windy also provides some very useful data for both present and forecast winds. Foreflight has useful wind depiction features if you have the required subscription. Winds on any summer day in the Midwest can change abruptly and often.
Most of my 172 experience has been in the S model. The airplane I scheduled that day was an R model. I never took the time to study the POH for the R model as I had the S model and for every other airplane I have flown. I just assumed – you know, it’s a Skyhawk. 20 fewer horsepower, different stall speeds, different white arc, different RPM range. Singly, those minor differences are probably not enough to give a pilot trouble. But all stacked up and in a moment of emergency, they can cause chaos.
I had flown that airplane before and decided that I didn’t want to fly it again. I should have taken some time to think through the reasons why I walked away from my first encounter with a negative impression. I had apparently become complacent to the point of ignoring my own assessment of the aircraft.
Given what the controller in the tower had told me about wind variability, I should have recalculated the potential crosswind. The METAR values indicated a crosswind component of 13 kts. from the left. The extreme of the variability of 180 degrees rather than 210 degrees, pushed that number to 17 knots. 17 kts. of gusty crosswind in a 172 is approaching the limits of my personal comfort level. But the air was smooth and gust free. (In fact, the NTSB report states that the wind was 19011G17KT at the time of the accident.)
We all were taught to add half of the gust factor to final approach speed. The gust factor would have been about 9 kts., so I should have carried an extra five kts. on final. But, I reasoned, I was experiencing no gusts, everything looked and felt right, I had the glidepath and was right on my planned speed.
The data shows I was 30 feet above the runway with a ground speed of 58 knots. I had 20 degrees of flaps. Power had just been set to idle. The airplane suddenly was blown to the right of centerline by a strong gust. I immediately put in left aileron and worked the rudder to get back to centerline. Just as abruptly the gust was gone, and I felt a sensation that I had not felt before in an aircraft. The left wing simply stopped flying – as if there was no lift at all. This did not develop like any stall I had ever experienced. It was not progressive, but rather sharp and sudden. The left main contacted the runway which induced a significant bounce and propelled the aircraft to the right. I always have my hand on the throttle during landings and my first instinct was to go around. Just then the right main made contact and induced a second bounce. I made the decision in that split second that adding power to what was now an out-of-control scenario would be a bad decision. I worked the control surfaces and brakes as best I could and aimed for the grass. The second bounce led to a flat landing, the nose gear collapsed, the prop hit the runway, and you can guess the rest. At some point I had pulled the fuel cutoff, killed the mags and master, and opened the door.
All my mistakes that day stacked up and, in the end, my skill as a pilot was not sufficient to overcome them. I had forgotten that gusts reported on the ground coupled with smooth air just above meant that I should expect wind shear. I didn’t consider that 400′ bluff just south of the field. Gusting surface winds from the south drop off that bluff and cause all manner of havoc. On that day, during that landing, that havoc came in the form of low-level vertical wind shear.
Would the extra 5 kts. of speed on final have changed the outcome of that landing? Maybe. Should I have executed a go-around? Affirmative.
The sounds that filled the silence after the engine quit will stay with me forever. An airframe does not bend silently; the fuselage does not slide on asphalt quietly; and the violence of that landing will forever be a part of this Airman’s psyche.
- Multiple mistakes were too much to overcome - May 3, 2023
- Learning to fly the wrong way—and loving every minute of it - January 22, 2020