Doing a proper pre-flight is a crash/don’t crash decision you make every time you fly. Here Jim Davis tells four stories of disaster, or near disaster, caused by pilots failing to check various tiny orifices.
On October 2nd 1996 a Peruvian Boeing 757 flew into the sea killing all nine crew and
61 passengers. The crash started when the aircraft was washed. The washing team had correctly covered the static vents on the bottom of the fuselage with masking tape so that they didn’t get water in them. However the team forgot to take the tape off afterwards.
The fatal flight was at night. The first officer, who did the pre-flight with a torch, failed to notice the tapes.
The final link in the chain is something of a mystery to me. Blocked static vents would initially cause the ASI, VSI and altimeter to under-read. I have no idea how much these errors would be. Theoretically the ASI should not have indicated any airspeed so they would have aborted the takeoff. But this was obviously not the case – mainly,
I believe, because the air inside the static tube is compressible.
Subsequent events and instrument readings were so bizarre that I think the only explanation for them is that the tape acted as a one-way valve letting air out of the system and not in. I say this because the altimeter was showing a safe height when the aircraft first hit the water.
CAPTION: This is what killed 70 people on the Peruvian 757.
Here’s part of the accident report:
Shortly after take-off just past midnight, the crew discovered that their basic flight instruments were behaving erratically and reported receiving contradictory serial emergency messages from the onboard computer, such as rudder ratio, overspeed, underspeed and flying too low. The crew declared an emergency and requested an immediate return to the airport.
Faced with the lack of reliable basic flight instruments, constantly receiving contradictory warnings from the aircraft’s flight computer (some of which were valid and some were not), and continuously believing that they were at a safe altitude, pilot Eric Schreiber and copilot David Fernández decided to cautiously begin the descent for the approach to the airport.
Since the flight was at night over water, no visual references could be made to convey to the pilots their true altitude or aid the pilots in the descent.
Also, as a consequence of the pilot’s inability to precisely monitor the aircraft’s airspeed or vertical speed they experienced multiple stalls resulting in rapid loss of altitude with no corresponding change on the altimeter.
While the altimeter indicated an altitude of approximately 9700ft, the aircraft’s true altitude was in fact much lower. It struck the water approximately 25 minutes after emergency declaration, and for 20 seconds the pilots tried to make the airliner climb. The airliner then crashed into the water. All nine crew members and 61 passengers died. The static ports are critical to the operation of instruments that show the pilots airspeed, altitude and vertical speed. They also provide additional functions such as warnings when flight characteristics approach dangerous levels.
The blockage of the static ports is regarded as one of the most serious faults that can occur within the avionics systems. As a result, the basic flight instruments relayed false airspeed, altitude and vertical speed data. Because the failure was not the instruments but rather in a common supporting system, thereby defeating redundancy, the altimeter also relayed the false altitude information to the ATC, who was attempting to provide the pilots with basic flight data. This led to extreme confusion. The fact that the flight took place at night and over water thus not giving the pilots any visual references was also a major factor.
CAPTION: This diagram explains how pressure instruments work.
To me it’s amazing that they managed to keep it in the air as long as they did.
A happier ending
In the early 1960s I was working as hangar-boy for Placo, the Piper importers for Africa. We were based at Wonderboom, just north of Pretoria. My immediate boss was Zingi Harrison. He was the best and most knowledgeable pilot in the world. He wore a bow-tie at all times, and loved to tell a story. And when Zingi spoke it was always worth listening.
On this particular day the big boss, Mr Piet Van der Woude sent Zingi to Durban in a single Comanche to collect Mr Piet’s 80-year-old mother, his spectacular wife Myrtle, and two rugrats. Zingi left Wonderboom with minimal fuel for this 500nm trip. When he arrived in Durban he filled the Comanche’s six tanks before loading his precious cargo.
He was on his way back at flight-level 105 and half way across the mountains in that nasty bit where you are out of gliding distance from both sides. At this exact point the comforting rumble of the big Lycoming stopped without warning. Zingi was baffled. He hit the fuel pump and changed tanks, even though the right main, which he was using, showed almost full and had given good service for the last 20 minutes.
The way Zingi explained it, the telling took longer than the flight, but in the end Old Piet’s beautiful wife, aged mother and the sticky infants were none the worse for their experience. Here is what happened. When he refuelled in Durban, Zingi didn’t take any notice of how much fuel went into each of the tanks. Why should he? But if he had he would have seen that the right main, although nearly empty, accepted only four gallons before it was full. There was good reason for this.
A bug, in Pretoria, had made its muddy home in the breather for that tank. This meant that while Zingi was using it on the outward flight the bladder fuel-cell was being scrunched into a little ball as the fuel was sucked out of it. Obviously at the pumps it took very little fuel to fill this now prunelike cell. When Zingi peered into it during the pre-flight inspection the tank looked full, and the gauge showed full because the float had been carried to the top with the scrunching.
This is not just a long-ago story about a guy with a funny name and a bow-tie. Mud-bugs are still around today, and so are bladder-tanks – it can happen to you tomorrow.
The lessons are plain. Make sure that what goes into the tank seems sensible. When you check for bugs – really check. And make sure you understand the system – some aircraft have only one breather for all the tanks, so you may not be as lucky as Zingi.
Battery acid blues
Not long after this, Obie Oberholzer, who was the boss of the maintenance workshop, summonsed Zingi and myself down to his hanger. When we got there he pointed at a Piper Tri-Pacer that had come in for an MPI. He had stripped off a big chunk of the fabric below the cabin.
We didn’t need a microscope to see what Obie was pointing at. The main longeron of the fuselage had a 50mm gap in it. Where there should have been a tough tube as thick as your thumb there was nothing. It had been eaten away by battery acid. Only the fact that Mr Piper builds very strong aeroplanes had prevented it from disintegrating in the air.
How can this happen? Believe it or not – it was the pilot’s fault. His pre-flights had been a less than perfect affair. He hadn’t gone poking about underneath with a torch and a bit of wire. If he had he would have found three little pipes below the battery box. One facing forwards, one facing backwards and one straight down. The forward facing one scoops up air which circulates through the battery box and which eventually goes out of the backward facing one, carrying sulphuric acid fumes with it. The downward facing one is a breather which is connected to the top of the battery and allows for expansion and contraction of the acid.
CAPTION: The right-hand pipe scoops air into the battery box where it circulates before coming out of the left-hand one. The centre one is a breather to allow expansion of the sulphuric acid in the battery.
I can’t tell you which of these three holes was blocked with a bug’s nest, but the result was the acid had eaten through the box and then through the airframe.
Tape and fate
Earnest Gann, in his wonderful book Fate is the Hunter, tells of his mate McGuire flying a C-87, a cargo version of the B24 Liberator. It was during WWII and they were flying over the vast white wastelands of Labrador. He was part of a search for another comrade, O’Connor, who had gone missing in a snowstorm in a similar aircraft some days earlier.
Each of the search aircraft had to cover a huge area, and was therefore pushing its range to the limit. Their biggest problem was navigation. The country is immense, sparsely populated and largely uncharted. The maps proclaimed their ignorance with enormous tracts of blank paper marked “Unexplored”. Such regions often contained whole mountain ranges and deep river valleys that were simply unknown.
As if things were not difficult enough, there was an almost perpetual low overcast which kept them pinned down and made astro-nav impossible. Finally, their compasses were virtually useless. The needle would sit for long periods on one heading and then slowly drift 30 or 40° to one side or the other. The problem is that the closer you get to the poles the weaker the horizontal component of the earth’s magnetism becomes.
This seems strange, but it’s because as you approach the pole the needle tries to move down to point at the pole. This downward force is known as component Z. As it gets stronger the horizontal force reduces, which means that your compass has very little inclination to tell you where the pole is.
Anyhow, McGuire had been flying for almost half of their endurance when his engineer noticed that two pieces of masking tape, which were used on the ground to prevent water freezing in the fuel tank breathers, were still in place. They should have been removed during the pre-flight. The only reason the engines kept running is because the rubberised tanks were collapsing as the fuel was sucked out. The same as Zingi’s Comanche.
O’Connor knew they were in serious trouble. The engines could stop at any moment as the tanks refused to collapse any further. Then he would not have enough useable fuel to return to base, or to any known alternate. While he was pondering this, “… one engine backfired and quit without further apology. Shortly afterwards, the second engine also ceased to function”.
He eventually contacted a half-built field on the north shore of the St Lawrence River. Here were his problems: the field was too short for a C-87; it was at night; and there was a snow storm and visibility varied from half a mile to almost zero. The ceiling was indeterminate – but way below limits for a range let down. Runway lighting was a few paraffin lamps and the wind was blowing hard across the runway. And they were flying on the two left engines only.
O’Connor was able to see the bits of tape that were trying to kill him. They were only a few feet away, but entirely unreachable. Through enormous skill, incredible bravery and a touch of luck he managed to land without damaging anything.
The pilots in that era were tough. Incidentally, Gann’s book starts with a list of 430 names of airline pilots - “comrades with wings forever folded… their fortune was not as good as mine”.
The holey word
So we have the 757, the Comanche, the Tripacer, and the C-87 all in serious trouble because the pilots didn’t pick up tiny holes that were blocked. Perhaps next time you do a pre-flight you will give a thought to these pilots and the little holes they missed.
CAPTION: Check with someone who knows that yours is the latest in Cessna's series of modded fuel caps.
If you fly a Cessna make sure it has the latest fuel caps. There have been countless mods, some leaked, some let water in and some refused to breathe. If you fly a Cherokee have a close look at the cap and see if you can figure out how it breathes. If you can’t then ask someone – it is not at all obvious.
CAPTION: Piper Cherokee fuel caps such as this one give no trouble, but see if you can figure out how it breathes.
Just to finish off, here’s a piece of wisdom by an unknown author:
We should bear one thing in mind when we talk about a pilot who has crashed. He called upon the sum of all his knowledge and made judgements that he believed in so strongly that he knowingly bet his life on them. That he was mistaken in his judgements is a tragedy, not stupidity. Every supervisor and contemporary who ever spoke to him had an opportunity to influence his judgements.
So a little bit of all of us goes in with every pilot we lose. Have fun and fly safely.
Jim Davis has 15,000 hours of immensely varied flying experience,
including 10,000 hours civil and military flying instruction. He is an
established author, his current projects being an instructors’ manual
and a collection of Air Accident analyses, called Choose Not To Crash. Visit Jim's website by clicking here.
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