BOAC, as promised, here’s an excerpt from one of the engineering discussion papers that I have. I’ve had it translated from Greek, and I’ve removed any direct references to the Cypriot maintenance team.
I know that some of the technicians involved in the engineering discussion are readers of PPRune, though not necessarily posters. It would be extremely valuable to our discussions if they, or others who have raised similar issues directly with Capt. Tsolakis and his team, would post to this thread, rather than simply read it.
Also worth noting is that the last news article that I posted states that the Preliminary draft report will be released before the end of March, but what was not said is whether it will be released to the general public at that time. I have a sinking feeling that, unless some pressure is brought to bear by those outside the investigation (particularly by the press), we will continue to be kept in the dark. And, if the cause of the crash is directly related to some technical occurrence, such as that outlined below, then withholding the report would serve no one.
AIR-GROUND SENSING
1. This is a very important system because it affects many other components of the aircraft, from the brake system to the engine power system, the APU electrical supply, as well as the pressurization controls. Ground-air sensing derives from two sensors, one on the right landing gear and another on the nose landing system. Electrical signals from the two sensors are distributed through 10 electronic cards from where they lead to various electrical and electronic aircraft modules.
2. It is possible to have an air-ground sensing problem as a secondary effect from related electrical wiring or electronic cards, rather than from the sensors themselves. A problem in the sensors leads to several malfunctions, which are immediately noticeable, so it’s more likely that there was a chronic and insidious secondary malfunction in the ground-air system, which may occur intermittently during flight and is difficult to trace by the engineers when the aircraft is on the ground [text removed].
3. Let’s assume that the electronic card, which prompts or cuts the T/O Warning Horn from the Aural Warning Generator (located above the captain’s left knee), had a malfunction, which retained the T/O Warning Signal to the Aural Warning Module instead of canceling it when the aircraft is in the air. One possible reaction of the flight crew, who has experienced the problem before, would be to pull the T/O Aural Warning CB. But, because this fuse is located right next to the CB of the ground-air sensor system, they could pull the latter by mistake. This could then cause serious problems to the pressurization system. I have seen this happening many times and the reverse is also likely: instead of the ground-air CB, they pull the T/O Aural Warning CB. This move would silence most of the flight deck’s aural warning systems, especially the cabin altitude horn, which sounds when cabin altitude passes 10,000 feet.
4. The situation becomes even more complicated given that the crew carried out a bleeds-off takeoff, which is often done in hot weather, with full loads and transit trips with no enroute refueling, as was the case with Helios Flight 522 to Prague via Athens. This was also the case in the five previous incidents of electrical malfunction in the aircon packs, which the same plane had experienced before its final flight. In this case, the APU compressor normally supplies bleed air for the initial cabin pressurization to allow the engines to develop full thrust in the early stages of climbout to a maximum of 17,000 feet. But the APU electrical generator is one of the systems directly affected by electrical malfunctions in the ground-air sensor system. So you can have a situation where APU electrics are also left ON together with APU bleeds without the crew knowing about it. More likely, however, is that the plane was left in AIR MODE on the ground after the pressurization test carried out three hours before the last flight. Combined with a secondary electrical fault in the ground-air sensor, this would affect the pressurization AUTO controls, outflow valves and take-off configuration warning systems in all sorts of unpredictable ways.