Weight Restriction or something...

[ILS26L]

Just wondering if someone can explain something to me... I was due to fly on BA242 from MEX to LHR but was refused boarding (although there were seats available) due to weight restriction apparently. Was told by ground staff in Mexico that the temperature was too high according to what they expected and some explanation about mexico city aiport being at 7,000ft, etc... To be honest I didn´t quite understand, and was a bit worried that this wasn´t completely true...
Can anyone try and explain what they think might have happened ..
many thanks, look really forward to reading on this subject.
Regards

[phoenix son]

ILS26L,

In a nutshell, what you were told was correct. The hotter it is, and the higher the elevation of the airfield, the more degraded the take-off performance is for any aircraft. This means that to achieve satisfactory take-off performance, a compromise must be met where either fuel or payload is left behind. Since there will be a minumum fuel requirement on the Flightplan that must be carried, that only leaves the payload, which on this occasion was unfortunately YOU! This is why generally you will find that "hot and high" airfields have long runways, in the case of Mexico City 12795ft or 12966ft depending on the runway in use.

PHX

[ILS26L]

Many thanks...
much appreciated, very interesting indeed !

[JW411]

That is exactly why Aeromexico and Mexicana were the only customers to buy the DC-10-15.

This was basically a domestic DC-10-10 with the much larger engines from the DC-10-30. It could depart Mexico City in the height of summer with a full load of passengers.

For most normal aircraft the problem is known as the WAT Limit (Weight against Temperature). This basically means that although the runway is more than long enough for the aircraft to get airborne, the engines are not powerful enough at that height and temperature to enable the aircraft to climb.

If you don't have a DC-10-15 or similar then you have to reduce your payload and chuck the punters off!

[Mad (Flt) Scientist]

I've always understood "WAT" to be "Weight, Altitude, Temperature", since, after all, altitude can be a pretty important factor.

[Teroc]

Thats my understanding of 'WAT' as well. Weight, altitude, temperature.

Of course this has now been changed to Mass, Altitude, temperature.....as we'll all be wearing spacesuits when we go flying soon...ahem !!

[boeingbus2002]

Why couldnt they accept the Pax/payload and make one stop in US or Canada for fuel?

I appreciate there would be extra costs incurred, but wouldnt the costs incurred to accomodate the pax and the reputation be greater?

Just curious!

[Rainboe]

Certainly not! It is not cheap to land a 747 en route (landing fees), have staff standing by to service it (turnaround fees), load on extra fuel (fuel costs- the operation is not as efficient and overall fuel will be greater), pay extra crew costs (and possibly have the crew run out of time if delayed), lose aeroplane time, be delayed on your schedule.....all to save a few hotel costs when you can probably pass those pax over to the competition anyway? All for the very few occasions in the year when it will be critical? Think about it!

[Send Clowns]

The reason for the WAT (or MAT :yuk: ) limitation and take-off limitations being stricter "hot and high" is the low air density. Not good for engine performance or aerodynamic performance. The aircraft has to reach a higher speed (true airspeed, that is) before it can support its weight by lift and there is less thrust available to reach that speed, so take-off performance is degraded. Then in the climb the thrust margin (thrust available compared to drag straight and level, i.e. the extra thrust available for climb) is lower, as there is less thrust. Thrust margin determines maximum angle of climb.

[ILS26L]

Would you therefore consider MMMX to be hazardous due to these conditions ? I suspect the fact that it's got a lake on one side a buildings on the other doesn't help either...

[Rainboe]

Would you define 'hazardous'? What are you saying- that because Mexico City is at some 8000', it is in some way 'hazardous'? Why? That is the reason for WAT limits possibly reducing the allowed weight of the aeroplane. The WAT limit is to compensate for the altitude of the place. What hazard is there?

[Mad (Flt) Scientist]

Mexico is not 'hazardous' by the specialist meaning in aviation safety. But it does have a reduced margin of safety.

Here you get into the actual practical intent of the regulatory performance minima, and their logical basis.

Most of the minima have NO BASIS IN LOGIC. There's no special reason why we expect certain minimum climb gradiets, or stall speed ratios, or runway deviations following an engine failure. The minima have evolved as a result of operational experience and, sadly, lives lost.

The numbers we have today are based on "these numbers have worked well for the last X years", "worked well" meaning there's not a smoking hole at the end of the runway (usually). But there's no evidence or analysis to actually prove that WAT-limit plus 50lbs is "unsafe" and WAT limit minus 50lbs is "safe".

Given that the minima are, ultimately, practical and impirical values, one must then consider where the justification for the minima comes from. Essentially, it's from keeping the overall hull loss rate below 1/10^7 or so, and keeping the newspapers headline-free. Since the majority of aircraft operate below WAT limits, much of our operational experience does NOT validate the WAT limit numbers, and our impirical pool of knowledge of the effectiveness of the WAT limit philosophy is less deep than we think. It may well be that the "overweight" operations at certain airports are being drowned out in the noise of the "low eight" operations world-wide. There aren't enough engine failures at the WAT cases to compound with loading errors, airframe damage or deterioration, pilot errors, etc., to reall test the WAT limit philosophy.

Of course, as more and more airframes and engines use FLEX-type takeoffs, we are gaining more experience at the WAT limit. And we may well find out, in due course, that the WAT limits really are not stringent enough......

[P-T-Gamekeeper]

In the RAF truckie world we often operate at MAT limits in some of our luxurious? destinations these days.

Have once, a long time ago, been forced by an old bold captain taken off above MAT limit. The result - as we turned at 500' to avoid terrain, the a/c lost altitude, and I lost any respect I ever had for that skipper.

Most weight limits follow a linear extrapolation, and so an excedance has a linear degradation in performance. If you exceed your MAT, it doesn't matter how long your runway is, your a/c cannot physically give you anymore bernoulli's.

[Continuous Ignition]

JW411,

My last outfit had 4 of those DC-10-15's. Man what a great airplane. It could almost be considered a STOL at lighter weights.

The only bad thing was the lack of range for when we would take it across the pond. If I remember we could only get about 6.5 hrs range out of them at MGTOW of 455,000 lbs

There was only one or two operating in Europe if I remember correctly.

Sadly what remains of Sun Country's -15 fleet can be found at the scrap yards around the US along with the fleet of Super 27's we operated.

Sorry for the thread hi-jack..

Now back to your regularly scheduled programming!

[Old Smokey]

ILS26L,

Hazardous ? - NO! It would have been hazardous if the airline had overloaded the aircraft.

Speaking in more general terms - Is Mexico City a more hazardous airport than, say, a comparable airport at Sea Level ? More hazardous, Yes, but not hazardous per se.

At a high altitude airport, the aircraft will often times be operating all of the way to the performance limit, thus, the existing performance is safe, but no additional performance reserve exists.

On the other hand, at a low altitude airport, particularly in cooler temperatures, the aircraft will reach it's Structural Limit before it's Performance Limit, thus, the performance available will exceed that required - An increased safety margin.

Regards,

Old Smokey

[bypass]

The very wonderful basics of those days of understanding Performance limited Take off Mass, and Structural, like good Ol Jo'Berg in Africa at a high field elevation with high temperatures, or Bankok Airfield too, Madrids at Two thousand odd feet too, Heavy, Hot and High elevation....no good, no density in the air, Cold, Light and Low Elevation....Great for those engines.

[bealine]

Slightly OT, I remember a few years ago a charter aircraft slewing off the runway at LGW and we had to operate using the shorter "Emergency" runway.

It was a nightmare explaining to passengers that the flights were operating with only half the seats (okay 2/3rds) in use because of the weight limitations, but if we hadn't, our BA operation at Gatwick would have come to a standstill!

It was sacrilege, too, to turn stand-by staff away when we knew there were seats available!

[Captain Airclues]

boeingbus2002

Airlines will generally not offload revenue passengers due to weight restrictions, but will reduce the allowable cargo load for that flight. However, I suspect that ILS26L might have been on a staff ticket. When booking a staff ticket the small print states that you will be onloaded after ALL revenue payload.

Airclues