May 9, 2015
OK so a number of times when I have been flying N844X I have been flying along (usually climbing) and heard a sudden BANG!
It is like a shotgun going off in the cabin.
Amongst the sound of the turbine screaming, the prop thrashing, the air rushing, and the air conditioner conditioning, and the air traffic controllers controlling, all coming through some noise-cancelling headphones that try to BLOCK most of that stuff OUT, it is actually sort of hard to figure out what the sound is, and where it is coming from.
It SEEMED to be coming from the main door (to my left), or left wing root or left main landing gear area, but I could not be sure.
But, suffice it to say, when about half the time you fly the airplane, there is a huge BANG coming from somewhere in the structure, you tend to worry a little.
I asked around about what might cause this, and got two ideas back:
1: It is little bits of carbon fiber strands breaking loose that are IRRELEVANT to the aircraft structure: Just some loose carbon fiber strands that got some resin on them and popped as the fuselage (naturally and safely) expanded under pressurization loads. It happened several times, though, and seemed a little loud and violent for just a few strands of carbon fiber, and that could not possibly continue to happen multiple times, every few flights or so. I mean: Once the strand is broken, it is not going to snap AGAIN on the next flight!
2: It is a compressor stall: Perhaps the engine was not overhauled quite perfectly for some reason (I have an overhauled PT-6, not a new one), and the airflow across a compressor stalled briefly. I am told this sounds like a shotgun going off. This was a scary prospect, because what it my engine was compressor was stalling itself, possibly damaging the engine? This is especially scary because of the cost of the engine (many hundreds of thousands of dollars!!!) and of course the engine is a little bit safety-critical. I watched the engine instruments intently right after it happened, though, and saw ZERO changes in ANY engine instruments! So how could it be a compressor stall, which would surely give SOME sort of engine instrument indication! (right?)
Well, the DOORS in the Lancair Evolution are “finicky” in the extreme. They are held in place in flight by seven latches that reach out of the door and grab the airframe.
Here is the scary part:
1: Each latch better apply about the SAME amount of tension, or one latch will bear TOO MUCH load while the other does not bear enough, which is not good structurally, and might not hold the door totally closed (!)
2: Each latch better latch ALL THE WAY, or the pressurization load will force the latches OPEN in flight (!)
3: You have to ADJUST all those latches to get them to bear the same amount of tension and close all the way by adjusting a complex and finicky system of gears and chains inside the door itself! Any mistake could violate one of the two rules above! ARGH!
Well, a few days ago, returning from interviewing a Patent-Troll victim in Florida, I noticed that the airplane was holding almost no pressure: I had to descend to 12,000 feet to continue a flight since the cabin would hardly pressurize! Careful inspection of the latches revealed: That rule #2 above was being broken! Three of the latches (the AFT ones) were not all the way latched, so air was leaking out of the cockpit through a crack in the door! ARGH!
This caused me to land, beg for some tools, and re-adjust those latches to be very sure that they all grabbed properly, and really look carefully at each latch.
Well, when time to go flying up to Connecticut to interview another Patent Trolling victim (I am using N844X as a mobile producer delivery-platform now, flying all over the Country interviewing victims of Patent Trolls) I looked at all the latches to make sure they were latched. ONE of them was about 1/16” from being fully-latched, which I thought would still be just fine. Well, climbing through 14,000 feet: BANG! The sound DEFINITELY was coming from the door, not the engine! I could FEEL the door jolt under my left arm, which was on the armrest built into the door! I quickly put the autopilot in altitude-hold to avoid any further climb and pressure-loads, and carefully inspected all the latches on the door (well, as carefully as you can while going 300 miles per hour in turbulence and air traffic control is telling you to contact another frequency, which, trust me, happens really fast in that airplane). All of a sudden, it became obvious: All of the latches on one of the chain drives were TOTALLY unlocked! The entire front of the door was getting no strength from the latches at all, and only the bottom and aft parts of the door were being held closed!!!! The BANG sound was obviously the latches violently popping from the latched to unlatched position due to the pressurization loads.
With simultaneous relief at knowing what that BANG noise was that I had been hearing (NOT the engine, thank goodness!), and fear that I was about to lose the door in flight, I turned off the pressurization system (less air loads on the door), descended to below 12,500 feet (no problems with hypoxia), reduced power (less speed and prop-wash blowing over the door, which does indeed lower pressure around it, I assure you! It is the Bernoulli thing thing), and told ATC that this show was over: I was turning around and heading right back to KCAE.
Upon landing, more than a little frustrated at the ongoing door problems, I broke out the tools, and got right back to adjusting the chain drive: This time to the FORWARD latches. With a bit of chain-length adjustment with some tighteners, I got the front latches to go ALL THE WAY closed (over-center, where increasing pressurization loads pull the latches even TIGHTER!) and put away the tools and tried again: Off to Connecticut at 28,000 feet, with the latches clearly staying over-center all the way, and everything now working perfectly.