I was flying from Heathrow to Miami last wednesday (AA 39) and for the first ⅔ of the flight we were flying at 28.000 feet, only afterwards did it go up to 33.000 feet. So far I was under the impression that for reasons of fuel efficiency, airliners would try to fly as high as possible, and looking at the same flight on different days, it seems like they do in fact usually fly higher. So what could have been the reason for such a low altitude? Was it just weight due to being fully loaded, or are there other reasons as well?
Looking at the route your plane took, this low altitude flight was likely to avoid the polar jetstream. This is a band of wind that blows west to east in roughly the area the first half of your flight was flying through. It exists mostly between 30,000 and 39,000 feet altitude, and can range from about 60mph to over 200mph. I suspect that the low altitude was to keep the plane below this jetstream, avoiding the headwind and fuel and time costs that flying through it would create. To answer the other part of your question, as I mentioned in the comments, weight shouldn't have been an issue because the plane you were in (a 777-300ER), has more than enough engine power to maintain a higher altitude, even fully weighted.
There are generally 3 reasons to pick lower altitudes:
- At departure your weight would be too much to climb to the desired altitude. Some aircraft are restricted by weight to get up to higher altitudes, usually a thrust-weight issue.
- Something's broken on the aircraft requiring it to fly at a lower altitude for safety reasons. As an example, the Air conditioning packs, that control the pressurization on an aircraft, if one is out, the flight might be restricted to a lower altitude so the system can handle the reduced output of the remaining packs to maintain a suitable pressure onboard.
- The most likely reason though in this case is to be at an altidude where the headwinds are less than what they are higher. Oftentimes the highest winds in the jetstream start in the FL300+ range, and run generally West-East. So trying to pick an optimal altitude is a factor of getting high enough for efficiency, but not to where you'll be losing that efficiency due to the higher winds aloft.