The 737NG takeoff performance manual seems to cover the 'hot' part of the 'hot and high' performance limitation equation, but the 'high' side seems left wanting. Sure, I can look up the effect of altitude on V speeds and expected takeoff thrust, but they don't tell me whether I can takeoff with a margin of safety.Can anyone advise me how to determine the aircraft weight limit for a high altitude (as in > 7000ft) airport, given runway length, runway altitude, flap setting, temperature and wind component? Gary

There are very few airports that 737s actually attend in this world that are over 7000 ft elevation. Airports like Albuquerque and Denver already have much longer runways than average, and there aren't that many major airports with big runways and elevations over 7000 ft. For those that do exist, special performance tables are created for airlines. All of these airports are in mountainous terrain and will most likely require all kinds of procedures for terrain avoidance. So there, your weight restriction and V-speeds are most likely limited due to obstacle and climb clearance and calculated figures from a standard performance table will not do.Actually, for many airfields, obstacle and climb clearance will cause different weight limitations and speeds, so the figures you may calculate from the performance tables are in fact incorrect. Real world airlines have one or more tables for every single runway at every single airport they attend (yes, that's a LOT of tables!).The maximum field elevation where any 737NG can depart from is 8400 ft.Iz

Thanks for the response Iz. This issue has risen a few times during the around the world trip that I am currently completing. The greatest extreme that I have experienced so far was at La Paz in Bolivia (SLLP), where the runway elevation is 13313ft. I know this altitude is beyond the 737NGs rated altitude limit, but I'm sure this airport has seen a 737 or two in its time. Thankfully, SLLP has a 13116ft runway and I was taking off at only 125Klbs gross weight, but I found there was no way to calculate whether I would make it into the air. In fact, I ended up using the acceleration vector on the speed tape in concert with where I was visually along the runway to give me a feel for whether I'd make it or not. The take off turned out to be uneventful, but still left me wanting for some sort of calculated comfort for the future high altitude airports that I know I will come across in my future travels.I guess this is not to be!Gary

Hi Gary,I wouldn't doubt that there may have been a 737 or two at that airport (wow, that's high, by the way), but I also know that not everything that flies there is flown by the book (or abiding the laws and regs for that matter!).The major issue here is most likely tire speed. Normal 737 tires are rated at max 194 knots (225 mph) ground speed. Now you already know that GS is a resultant of TAS and wind, and for a given IAS, the TAS goes up with altitude. Since TAS = GS when on the ground, rotation and landing speeds may exceed the max rated figure by a lot and even though the tires may hold, you simply would be illegal to operate there.Sure, the aircraft could take off from that elevation: It is also to climb at 13000 ft, and quite well actually. The other thing to consider is n-1 (engine failure) situations. What if you're heavy, it's hot and you lose one engine at V1? Can you clear all obstacles? Sure, your two engine takeoff went fine by the seat of your pants, but would you meet all obstacle clearances? And by that, of course, I mean planning wise and by set margins, not just clearing the mountain top by three inches.The reason for V1, V2 and to a lesser degree, Vr calculation are exactly this: Safety in case of engine failure at the most critical time of the takeoff (@ V1). And at these very high altitude airports, that connects right back to what I said earlier: What if you lose your engine just before V1 and your tires are over max rated groundspeed, what would happen? Braking capacity is degraded on one or more burst tires, to state it mildly.Have fun,IzBy the way: It may have been the case that these 737s had official exemption, higher speed-rated tires or were test-flown or were on experimental status.

Hi Iz,At least one 737 model, the -300, has been certificated to operate into (and out of) La Paz. There is a special high altitude appendix to the Airplane Flight Manual (available for purchase from Boeing, I'm sure) for this operation. Tire speed limits (225 mph), as for any operation, must be obeyed.See, for example, http://www.b737.org.uk/lapazairport.htm for an interesting description of Varig's operations into La Paz.There are other examples of high altitude operations, like the 757 at Lhasa in Tibet at 11,621 feet. Most of these operations required demonstration flights to either obtain airworthiness certification data for the Airplane Flight Manual or for operational approval, or both.Don S.

Excellent link thanks Don, including that website in general. Now I know why N1 was up around 85% at full flap on final, thus severely limiting my climb performance if I had to go around. Flaps 15 for such very high altitude approaches in future for me!Gary

Iz,I never considered the TAS effect increasing ground speed for take off at higher altitudes. Good point!Given the number of airports that I will be visiting on my trip, many in obscure, uncharted and in some cases navigation aid free regions, I will regularly have to use my "sim pilot judgement" to make go / no go decisions such as this. Ah well, it's all part of the fun. Fortunately, my trip so far has been incident/accident free after 54 airports, 33,362nm and 84hrs 39mins flight time (real time) since 8 Feb 04, but who's counting!Gary