Schools were being torn apart for ammunition to hurl at the police, who were replying with high-powered rifles. The roar of the aircraft's engine drowned out the sound of gunfire, but I could see the crowds forming and splitting in the veld below like cells under a microscope. In the driveway of the hated Bantu Administration Board, teenagers surrounded an overturned car, which suddenly burst into flames. It belonged to a white social worker, who had been dragged out and beaten to death a few yards away.
In the days and weeks that followed, hundreds more people were to die, nearly all of them young and black. But the uprising seized the townships of South Africa: the white authorities, who in the 16 years since the Sharpeville massacre had jailed or driven abroad the previous generation of black leaders, and had mounted cross-border raids to stamp out any attempt at guerrilla war, were now faced with an insurrection among blacks too young to remember the unrest of the early 1960s.
In his autobiography Nelson Mandela remembers the arrival of the first members of the new generation on Robben Island: "These young men were a different breed of prisoner from those we had seen before. They were brave, hostile and aggressive; they would not take orders, and shouted 'Amandla' [Power] at every opportunity. Their instinct was to confront rather than co-operate." The new prisoners, he admits, "were almost as sceptical of us as they were of the authorities. They chose to ignore our calls for discipline and thought our advice feeble and unassertive. It was obvious that they regarded us ... as moderates. After so many years of being branded a radical revolutionary, to be perceived as a moderate was a new and not altogether pleasant feeling."
If 16 June 1976 - now commemorated as Youth Day - was a shock for the South African government, in many ways it was an equal shock for the ANC. The uprising posed the greatest challenge to the organisation in its history thus far, and the legacy of Soweto could be the worst threat to it in government.
The irony 20 years ago was that when Pretoria blamed "outside agitators" for the uprising, the ANC wished it were true. The organisation was struggling to maintain an effective presence in the townships, and its place in the limelight had been taken by proponents of "Black Consciousness", such as Steve Biko, who argued that blacks had to eradicate their own feelings of inferiority - the oppression within - before they could overcome white subjection. They preached a brand of black nationalism which rejected the help of other races or political creeds.
Far from being new, these views were an unhappy reminder of previous crises in the ANC. As Mr Mandela says, "They closely mirrored ideas I myself had held at the time of the ANC Youth League a quarter of a century before. We, too, were Africanists; we, too, stressed ethnic pride and racial self-confidence; we, too, rejected white assistance in the struggle. In many ways, Black Consciousness represented the same response to the same problem that had never gone away."
After they seized control of the ANC, Mr Mandela and most of his fellow Youth Leaguers abandoned black exclusivism, but the argument continued for a decade, leading in 1959 to the traumatic breakaway of the Pan Afr- icanist Congress (PAC). Now the ANC in exile was confronted with thousands of newly-arrived young radicals, espousing similar ideas. Many were contemptuous of their elders for having submitted to apartheid; some talked of forming their own organisations. To them Nelson Mandela, who had been in jail for 13 years in 1976, was just a name.
Co-opting these young lions, most of whom demanded military training, was a huge and sometimes violent task for an organisation whose morale and efficiency had been impaired by years of setbacks, including constant infiltration and betrayal. The sudden growth of Umkhonto we Sizwe (MK, or Spear of the Nation), the military arm, was the worst strain - many of the new recruits had been so poorly educated in the townships that they had difficulty absorbing training. Frustration and anger at primitive conditions brought revolts in the training camps in Angola in the early 1980s; the brutal treatment of alleged "traitors" in Angola is a scandal that still dogs the ANC.
The infusion of militancy, however, eventually made MK a force capable of exploiting the newly-favourable conditions within South Africa. One potential competitor for the loyalty of the townships had been removed by the South African authorities when Steve Biko was killed in detention. It is interesting to speculate whether Biko - who is not mentioned in Mr Mandela's autobiography - would have joined the ANC, as so many black consciousness followers did, or whether he would have remained aloof, but the ANC would have emerged at the head of the opposition to apartheid in any case.
Biko would have agreed, too, that the key strategy was to make the townships ungovernable. More than anything MK or Western sanctions could do, this finally forced the white government to negotiate, but it has left the new South Africa with its most serious problem.
Most of the generation of 1976 sacrificed their education, either as guerrillas or as "comrades" in the townships, where they enforced boycotts of the schools along with all the other trappings of the white state. Now in their thirties, they trail behind them millions of younger black South Africans who have unrealistic expectations and no skills, save those of "struggle" and "mass action"; the worst of them became addicted to intimidation of their fellow residents. The contrast between their situation and that of the whites made South Africa's epidemic of violent crime inevitable.
It is hard to see what was on offer for this class when South Africa's first black finance minister, Trevor Manuel, unveiled his maiden budget on Friday. Described as "market-friendly", it emphasised privatisation, the reduction of the budget deficit, a tight monetary policy and the relaxation of exchange controls. Although it set a target of 6 per cent annual growth by the turn of the century, with 400,000 new jobs a year, this will have no more than a modest impact on unemployment in the townships, which runs at 40 per cent - one economist called the budget "a defeat for the ANC's left wing".
The impatient young ANC activists who ousted their elders as the 1950s began, and managed to enlist their juniors in the 1970s, have this to ponder on Youth Day. What can they do for the next generation as it demands its turn?