Beirut — For four months since the Israeli invasion of Lebanon, the only planes to fly over Beirut were Israeli warplanes. Little wonder that the roar of passenger planes was a tremendous psychological boost.
The Lebanese capital was jubilant over the Sept. 30 reopening of Beirut international airport after Israel evacuated its last position there under the watchful eyes of United States marines.
The jubilation was tempered somewhat by reports later in the day that one marine had been killed and three others injured when some unexploded ordinance at the airport was detonated.
An encouraging development was the ceremony near Beirut's National Museum, one of several crossing points between Christian east and Muslim west Beirut, pockmarked and gauged by eight years of sniper and artillery fire during Lebanon's ongoing civil war. For eight years movement across these points was at one's peril. Sept. 30, surrounded by nervous Lebanese Army soldiers, President Amin Gemayel formally declared all crossing points open. Beirut, he said, was now a reunited capital - ''no more an east . . . and west'' - as a step toward reunification of all Lebanon.
But one day of celebration - for which Beirutis were long overdue - could not obscure the fissures under the facade of hope. ''Seven years of civil war can't be papered over quite so easily,'' murmured a Lebanese journalist.
Among the problems facing Mr. Gemayel:
* The two sides of the formerly divided city remain unequal in one critical respect. Israel's entry into Lebanon and to west Beirut led to the disarming of one Lebanese faction - Muslim leftists - and the expulsion of their ally, the Palestine Liberation Organization. But the Christian militias - grouped in the Lebanese Forces - retained their weapons.
Few west Beirutis regret the departure of bands of armed militiamen or the removal of tons of ammunition from basements, mosques, and schools by Israel and lately by the Lebanese Army. But in a country where one's militia has become the key to power, Muslims will remain apprehensive until the Christian militias are disarmed.
* The future of the Lebanese Forces, 12,000 strong and tougher than the Lebanese Army, is not yet clear. Devoted to their murdered leader President-elect Bashir Gemayel, senior military sources among them say they will not disband until his goal of driving all foreign forces from Lebanon has been achieved. At present this includes several thousand PLO men in east and north Lebanon, 30,000 Syrians, and Israel's Army in Lebanon.
Bashir Gemayel had planned to dissolve the forces and institute universal military service to strengthen the Lebanese Army. In the long run Muslim leaders will be watching to see if President Amin Gemayel can control the Lebanese Forces and achieve the same.
Unlike his brother, President Amin Gemayel had previously had little to do with the forces, and is not likely to press them at the moment. But he has talked of removing militias from all Beirut, including the east. The entire city would then be patrolled by the Lebanese Army, the police, and the multinational force.
* The future of the Lebanese Army, on whom President Gemayel - as well as the United States - is counting to enforce stability in Lebanon, is also unclear. Decimated by the civil war and virtually idle for the last seven years, the Lebanese Army has now moved into control of positions abandoned by the Israelis in west Beirut. Lebanese politicians are watching anxiously to see if the Army - beefed up by American aid money and training - can win public respect and absorb demobilized militiamen.
* Evacuation of foreign forces from Lebanon is a critical issue. Uniting Beirut is a snap compared to the task of ridding Lebanon of PLO, Syrian, and Israeli troops. Lebanon's leaders are gratified at President Ronald Reagan's pledge to keep US marines in Lebanon until all foreign forces leave the country. This is a bigger role than originally expected here for the multinational force.
But analysts here are dubious about how quickly this can be achieved, despite expressed Syrian and Israeli willingness to leave.
For one thing PLO leaders have expressed determination to keep east and north Lebanon as a base. There is no guarantee that Syria will press the PLO to pull out.
For another, Lebanese are worried about Israeli terms for withdrawing from all Lebanon. President Gemayel is not inclined to sign a peace treaty with Israel. But Israeli Defense Minister Ariel Sharon has said if Israel does not get a treaty, it will set up a security zone 50 kilometers north into Lebanon, in effect separating the area from direct control of the Lebanese government and leaving it in the hands of Israel's southern Lebanese ally, Maj. Saad Haddad.