I'm usually cold and then hot (or hot and hten cold) on planes. If the outside temperature is 18C or higher, then when you first get on the plane it can often be quite stuffy until they push away and start circulating air. If your flight is delayed on the ground, it can be very, very stuffy. (Once on a flight from Sao Paulo to Paris, we ended up diverting to another Brazilian airport because a passenger got sick and then died. We ended up sitting on the ground in a very steamy location for about 2 hours before taking off again.)

Once the flight gets going, I'm often chilly. There are blankets available, so this helps.

mmcd: I don't know if you're a man or a woman. I would advise a woman to wear two light layers (e.g. a twinset tank top and cardigan) and carry a pashmina scarf, which is quite versatile. The pashmina serves as the blanket for my top half on the plane (I never let airplane blankets near my face), a shawl to throw over my shoulders if I'm wearing a dress, a cover-up for my hair if I'm visiting a church or mosque and a throw blanket for naps in the hotel room. For men, I would also advise wearing two light layers, e.g. a tshirt with a cotton sweater, zip fleece jacket or something similar. Even though normal outdoor temperatures might be high in Manila, you might find your hotel room or another indoor environment over-airconditioned, so the light sweater could be handier than you think.