Apparitions and presences which display cognition are in many ways the most intriguing, that the human spirit can, in one way or another, survive death, and is the most rare. Accounts of interactive ghosts often appear to suggest that they have a purpose. Comforting the bereaved seems to be the most typical, but sometimes, their presence is more elaborate than this.

The L-1011 passenger plane was due for take-off, and the flight engineer was mid-way through carrying out the routine pre-flight inspection. 'You don't need to worry about the pre-flight, I've already done it,' Second Officer Don Repo told him. Under normal circumstances, the engineer might have gratefully received this comment. But Repo was not the appointed second officer on the flight. Nor, for that matter, on any flight. Don Repo was dead.

Don Repo and his colleague Captain Bob Loft are the most fascinating interactive, protective ghosts on record. Their story begins when their lives ended: on the evening of 29 December 1972 when Eastern Airlines flight 401 crashed in the swampland of Florida's Everglades, killing 101 people

The flight had been uneventful and was preparing to land when Captain Loft became concerned that the nose-gear might not have locked into place. Second Officer Repo, the flight engineer, agreed to carry out a visual inspection, and climbed into the 'hell-hole', a small compartment beneath the cockpit. There, he was faced with the sight of a watery marsh just below him - the last thing he saw before the plane plunged to its destruction.

It transpired that a far more serious fault had occured than Repo and Loft imagined. The plane's autopilot system had a mechanism which caused it to switch off if the steering column was pushed in a certain way. The autopilot had been set to keep the plane at an altitude of 2,000 feet, but as the men had moved about the cockpit, one of them had unwittingly disengaged the system. Within a minute, the plane had descended to just 500 feet above ground level, although the autopilot readout had continued to display an altitude of 2,000 feet. A warning signal had sounded, but was not loud enough to be heard by Loft through his headphones, nor, obviously, by Repo in the hell-hole. Twenty seconds later, it was all over.

Loft and Repo were among the dead, but the airline industry had by no means seen the last of them. Although Eastern Airlines refuses to discuss the matter, researchers have interviewed numerous individuals claiming to have encountered the ill-fated pair on L-1011 aircraft - and, oddly enough, usually on the very lanes utilising recycled parts of flight 401.

As the reports would have it, Bob Loft and Don Repo devoted their after-lives to watching over the passengers and crew of these little Lockheed passenger planes.

Many of the testimonies come from pilots, flight officers, and even a vice-president of Eastern Airlines, who allegedly spoke with a captain he assumed was in charge of the flight, before recognising him as the late Loft. Other sightings are convincing because they have multiple witnesses: a flight's captain and two flight attendants claim to have seen and spoken to Loft before take-off and watched him vanish - an experience that left them so shaken they cancelled the flight; a woman passenger made a concerned enquiry to a flight attendant regarding the quiet, unresponsive man in Eastern Airlines uniform sitting in the seat next to her, who subsequently disappeared in full view of both of them, leaving the woman hysterical. When later shown a sheet of photos depicting Eastern flight engineers, she identified Repo as the officer she had seen. But Repo and Loft are apparently not content merely to be present.

Often their style is far more hands-on - particularly in Repo's case. Aside from his appearance to a preflight engineer whom he appeared to have been assisting. There is a testimony from a flight attendant who observed a man in flight-engineer's uniform, standing by the galley oven, whom she later recognised as Repo. Also he has been seen in the hell-hole by a flight engineer who accessed it in order to investigate a noise coming from below the cockpit. On another occasion Faye Merryweather, a flight attendant, saw Repo's reflection from an oven in the galley of Tri-Star 318. Understandably alarmed, she fetched two colleagues, one of whom, the flight engineer, had been a friend of Repo's and recognised his description instantly. All three heard Repo warn them to 'Watch out for fire on this airplane'. The plane later encountered serious engine trouble and the last leg of its flight was cancelled. It is interesting to note that the galley of Tri-Star 318, had been salvaged from the wreckage of flight 401.

The sightings were all reported to the Flight Safety Foundation (an independent authority) which commented: 'The reports were given by experienced and trustworthy pilots and crew. We consider them significant. The appearance of the dead flight engineer (Repo) was confirmed by the flight engineer.' Later, records of the Federal Aviation Agency record the fire that broke out on the aircraft as they had been warned of from Repo.

Many of the accounts concerning the ghost airmen would seem to suggest that they have an agenda and purpose, one testimony spells it out clearly. An L-1011 captain reported that Don Repo had appeared and told them: 'There will never be another crash of an L-1011...We will not let it happen.'

There have indeed been no fatal accidents involving that model of plane since Repo's death in 1972.

If many allegedly true reports are to be believed, the old adage that dead men tell no tales is a fallacy. History has numerous stories of spectral entities helping the living to locate wills, deeds, treasure and such. Other accounts infer that 'get someone to discover my mortal remains' is at the top of many ghost's 'to do' list. Countless true hauntings begin with seemingly purposeful disturbances and end with the discovery of a skeleton concealed somewhere suggestive of a wrongful death. Borley Rectory in Essex, Britain's most notorious haunted house, had its mournful nun yearning for a sacrosanct burial after countless decades bricked-up in the cellar.

Sergeant Arthur Davies, 30-year-old leader of the English platoon stationed at Dubrach, Scotland, was last seen alive on the morning of 28 September 1749. Taking leave of his platoon, he set off into the hills for a spot of hunting and never returned. After four days, the search for him was abandoned, but foul play was not suspected given the treacherous Highland terrain: and what its crags, bogs and powerful water-currents could do and easily conceal a corpse.

In fact, Davies was indeed murdered and at the hands of Duncan Clerk and Alexander MacDonald, a deerstalker and a forester who's motives were wrapped up in the acrimonious atmosphere between the Scots and the English which had pervaded since the English brutalities of 1746. Without witnesses, a body or even a suspicion of murder however, the chances of the case ever coming to court would have been non-existent. But today, anyone caring to visit the General Register House archives in Edinburgh can peruse the Court Record Book for the years 1752 - 1754 and read about it themselves.

Ten months after the disappearance of Sergeant Davies, a young shepherd named Alexander McPherson, who lived a couple of miles south of Dubrach on the slopes of Christie Hill, had a very wierd experience. As he lay in bed, he saw the figure of a man who announced to him that he was Sergeant Davies and spoke to him at some length.

On 11 June 1754, McPherson took the stand in court, as a witness for the prosecution. An excerpt from the court transcript of the prosecutor's declaration reads �...the Deponent rose from his Bed and followed him to the door and then it was as he had been told that he said he was Sergeant Davies who had been murdered in the hill of Christie about near a year before and desired the Deponent to go to the place he pointed at where he would find his Bones...' The terrified McPherson followed the spectre�s instructions and found some skeletal remains not knowing what to do, he left the cadaver and fled.

The court records report that the apparition returned to McPherson's home on a second occasion, an account corroborated by Isobel McHardie, the wife of McPherson's employer, who was also present. This time, the spectral sergeant asked McPherson to give his remains a proper burial and to contact his friend Donald Farquarson. And for good measure, Davies named his murderers.

Farquarson�s court testimony asserts that he was initially sceptical of McPherson's story, but he allowed the young stranger to lead him to the remains which he positively identified as those of his friend, the hair colour and tattered remnants of clothing being a definite match. For many reasons - the atmosphere of political tension, the reluctance to admit involvement in ghostly goings-on and the lack of any concrete evidence being but three reasons to why they all decided not to report the murder. Although the men fulfilled Davies's wish for a burial.

Many months later, the court heard, McPherson was sacked from his job and found himself employed instead by Duncan Clerk, one of the men Davies had named. During a row with Clerk, McPherson accused him of murder, and was surprised when the man offered him twenty pounds to keep it quiet.

It was not long before village gossip reached the ears of the authorities, who brought the case to court, since, aside from the rather out-there ghost story, there was the more acceptable evidence of McPherson's hush-money (which had taken the very tangible form of a written and signed IOU from Clerk). Also there was the fact that two distinctive rings which had belonged to Davies were found on the fingers of Clerk's mistress. The authorities even found two witnesses to the murder. Although MacDonald and Clerk were eventually acquitted.

Ironically, it was not the legally weak ghost testimony which led to the acquittal, but a complex bout of hostility between the British prosecutor and the Scottish defence advocate which started with one of the witnesses being hanged for an unrelated felony and ended with charges of intimidation which brought the trial to a close.

Davies may not have succeeded in having his murderers put away, but he achieved the distinction of being the first ghost to have his testimony tacitly admitted as evidence in court.

But does this story, or any other like it, prove the after-life hypothesis? Not necessarily. As researcher John Spencer asserts: 'You could never absolutely say: this one proves ghosts must be coming from beyond the grave. There is always another possibility. Albeit another paranormal possibility. But there is always another thought.�