Is a composite key correct here?
Maybe the following approach would be of help (the parent tables Uld and Flight should be created first; test server running PostgreSQL 9.3)

create table Uld ( uldno varchar(10) primary key);

create table flight (flightid varchar(32) primary key);

create table LPN (
  id integer primary key
, uldno varchar(10) references Uld(uldno)
, flight varchar(32) references Flight(flightid)
, unique (uldno, flight)
);


Test data:

insert into Flight values ('A'),('B'),('C'),('D');

insert into Uld values ('AAA12345ef'),('BBB12345gh')
, ('CCC12345ij'),('DDD12345kl');

insert into LPN values 
(1, 'AAA12345ef' ,'A'),
(2, 'AAA12345ef' ,'B'),
(3, 'AAA12345ef' ,'C'),
(4, 'BBB12345gh' ,'A'),
(5, 'BBB12345gh' ,'D'),
(6, 'BBB12345gh' ,'C'),
(7, 'CCC12345ij' ,'A'),
(8, 'CCC12345ij' ,'B'),
(9, 'CCC12345ij' ,'C'),
(10, 'DDD12345kl' ,'A');


We get (something like):

+------------+
| table: Uld |
+------------+
| uldno [PK] |
+------------+
|AAA12345ef  |
|BBB12345gh  |
|CCC12345ij  |
|DDD12345kl  |
+------------+

+-------------------------------+
| table: LPN                    | 
+-----------------+-------------+
| id | uldno [FK] | flight [FK] |
+-----------------+-------------+ 
|1   | AAA12345ef |A            |
|2   | AAA12345ef |B            |
|3   | AAA12345ef |C            |
|4   | BBB12345gh |A            |
|5   | BBB12345gh |D            |
|6   | BBB12345gh |C            |
|7   | CCC12345ij |A            |
|8   | CCC12345ij |B            |
|9   | CCC12345ij |C            |
|10  | DDD12345kl |A            |
+-----------------+-------------+

+---------------+
| table: Flight |
+---------------+
| flightid [PK] |
+---------------+
|A              |
|B              |
|C              |
|D              |
+---------------+


Testing:

-- eg which Ulds are on flight A?
select uldno
from LPN 
where flight = 'A';


The data types for LPN.id and Flight.flightid should be adjusted as per requirements.  This layout should also work if there are several LPNs per uldno (if they have the same flightid in the LPN table).
It may aid your understanding of "keys" if you (also) regard them as "constraints" (~ rules that define certain limitations see eg https://docs.oracle.com/cd/E11882_01/server.112/e40540/datainte.htm#CNCPT322).