Not quite managing my first join

Hi!

I'm within a hair of getting my first join to work, but something is
wrong and for the life of me I can't figure out why. Hints, anyone?

====================================================================
select t1.month,t1.single,t2.multi from (
select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as single
from flights,aircraft
where flights.callsign=aircraft.callsign and aircraft.engines=1
group by month order by month) as t1
full outer join (
select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as multi
from flights,aircraft
where flights.callsign=aircraft.callsign and aircraft.engines=2
group by month order by month) as t2
on (t1.month = t2.month);

month | single | multi
---------+--------+-------
2003-08 | 5.6 |
2003-09 | 3.5 |
2003-10 | 4.2 |
2003-11 | 17.7 |
2003-12 | 16.4 |
2004-01 | 13.3 |
2004-02 | 1.7 | 7.6
2004-03 | 12.2 |
2004-04 | 13.3 | 4.1
2004-05 | 7.3 |
2004-06 | 7.1 | 9.4
2004-07 | 2.8 | 7.6
2004-08 | 6.0 | 7.0
2004-09 | 7.9 | 2.4
2004-10 | 2.5 |
| | 17.4
2004-12 | 5.8 | 3.1
| | 10.1
| | 14.1
2005-03 | 9.5 | 2.7
(20 rows)
====================================================================

The problem is that the query seems to be returning all the correct
results, *BUT* I have two problems:

- Instead of zero where I did not fly a multi-engine airplane, it
shows no value at all.

- For columns where t1 had no value, no month is shown.

How can I fix those two little details? My brain is temporarily fried
and I'm not understanding the docs very clearly.

Thanks!

--
Rodolfo J. Paiz <rpaiz(at)simpaticus(dot)com>