Collins lists the ‘about to‘ structure under adjectives, pointing out that there is no future tense in English. There are many ways we refer to the future in English. The structure means ‘very soon‘.
There are 3 almost identical B2 points in the category of FUTURE in the English Grammar Profile that could all be just one entry:
Point 41 in the past is defined as:
simple past form of ‘be’ + ‘about to’ (often with ‘just’) to talk about the immediate future from a point in the past.
Student example:
This man walked in our direction and just when I was about to run away from him,
he introduced himself very kindly and offered us some help.
PELIC Portuguese Female level 5 writing class
Point 43
future expressions with ‘BE ABOUT TO immediate future, often with ‘just’.
Point 44
future expressions with present forms of ‘be’ + ‘about to’.
An iWeb search for:
_vb about to *
1 IS ABOUT TO BE 10204
3 IS ABOUT TO GET 4958
4 ARE ABOUT TO BE 4004
5 ARE ABOUT TO GET 3245
6 IS ABOUT TO CHANGE 3152
7 IS ABOUT TO HAPPEN 3123
8 IS ABOUT TO BEGIN 2999
9 IS ABOUT TO START 2671
10 IS ABOUT TO GO 2555
was|were about to *
A2: become, enter, fall, happen, hit, lose, turn
1 BE 5486, 2 GO 2335
3 WAS ABOUT TO HAPPEN 1992
16 WAS ABOUT TO BECOME 882
17 WERE ABOUT TO GET 825, 18 WAS ABOUT TO COME 822, 19 WAS ABOUT TO HAVE 816, 20 WERE ABOUT TO GO 795, 21 WAS ABOUT TO ENTER 755
22 WAS ABOUT TO TURN 680
23 WAS ABOUT TO FALL 667
24 WAS ABOUT TO ASK 612, 25 WERE ABOUT TO LEAVE 604, 26 WERE ABOUT TO DO 524
27 WAS ABOUT TO LOSE 486
28 WAS ABOUT TO BUY 471
29 WAS ABOUT TO HIT 470
30 WERE ABOUT TO TAKE 470, 31 WAS ABOUT TO END 459, 32 WERE ABOUT TO START 437, 33 WERE ABOUT TO SEE 429, 34 WAS ABOUT TO THROW 414, 35 WAS ABOUT TO PUT 408, 36 WAS ABOUT TO BREAK 407, 37 WAS ABOUT TO EMBARK 383
38 WERE ABOUT TO ENTER 372
was|were * about to *
1 WAS N’T ABOUT TO LET 397
2 WAS JUST ABOUT TO POST 267
3 WAS JUST ABOUT TO START 252
4 WAS JUST ABOUT TO ASK 246
5 WAS JUST ABOUT TO SAY 242
6 WAS JUST ABOUT TO GO 212
7 WAS NOT ABOUT TO LET 194
8 WAS JUST ABOUT TO GIVE 162
9 WAS JUST ABOUT TO GET 147
10 WAS N’T ABOUT TO GIVE 129
11 WAS JUST ABOUT TO MAKE 123
12 WAS JUST ABOUT TO LEAVE 122
13 WAS JUST ABOUT TO TURN 106
14 WAS JUST ABOUT TO TAKE 103
15 WAS NOT ABOUT TO GIVE 102
16 WAS NOT ABOUT TO BE 98
17 WAS JUST ABOUT TO BUY 97
18 WAS N’T ABOUT TO GO 95
19 WERE N’T ABOUT TO LET 92
20 WAS JUST ABOUT TO BE 89
They‘d listened to songs of its origin from their childhood, and they weren’t about to give up on that without a fight.