Monday, January 23, 2012

Questions Answered : Weight restrictions and fuel

Question : Yesterday I was on a flight from Houston to Omaha and apparently the ground crew loaded too much fuel.  Consequently, we taxied out to the end of the runway, but then sat off to the side while the captain ran the engines up to burn off the excess.  We probably sat there about 15 minutes, and I couldn't help wonder why on earth didn't he simply fly lower or climb at a slower rate instead of making us sit there.  Is this technique normal???

Answer:

Yes it can be normal. Weight and balance is a tricky task at best..... aggravating at worst.

This is really good timing on your question as I've had 3 flights with fuel issues in the last 4 days. I'll write those blogs in the coming days. For now...I'll stick to the question.

The ground crew could have legitimately loaded the correct amount of fuel, but then conditions changed. Most fuelers are good so I will assume that's what happened. I will attempt to explain a flight where the "right" amount of fuel can be the "wrong" amount of fuel.

I will use the following numbers:

Empty weight:

Max Ramp Weight: 61000

Max Takeoff Weight:60750

Max Zero Fuel Weight: 51610

Max Landing Weight:54578

Empty Weight: 38500

Those are the easy weights. Issues come up with short runways or airports near terrain. Just because a plane can takeoff weight 60750 pounds doesn't mean it will be able to every time.

So in the original question "why on earth didn't he simply fly lower or climb at a slower rate instead of making us sit there"....here ya go.

Let's take a look at LGA....an airport with short runways and terrain (buildings). This flight will take place in the winter.

Winter operations cause performance penalties just like summer operations. The reason? Ice......and the need to get rid of it.

I will use the following weather:

KLGA 240051Z 17012G18KT 1/2SM R04/6000VP6000FT -RA BR OVC020 03/01 A2956 RMK AO2 SFC VIS 2

Low vis...rain and windy.

I'm going to keep it really simple (even though it gets incredibly complex when taking into account wind, pressure and temp). Runway 22 is in use for takeoff.

The flight has 58 passengers. Their weight is assumed to be 10324 (178 pounds each). Before boarding begins all are assumed to be an adult.

The 2 hour flight is expected to burn 6500 pounds of fuel

The FAA required 45 minute reserve fuel weights 1500 pounds

The destination weather is marginal. The alternate requires 2300 pounds of fuel. Total of 10,300 pounds of fuel.

So lets add it up....20624 pounds of fuel and people...add in the empty plane weight of 38500 equals 59124....leaving 1626 pounds to spare. We haven't looked at the runway performance data. My airline has big fancy computer program for this. The engineers look at accelerate and stop distances along with terrain avoidance (and all the other FAR required performance data). For runway 22 runway we are limited by the climb performance to avoid terrain. Max takeoff weight is 59800. Not a big hit....950 pounds.

Now for you fellow airline pilots I'm fully aware credit (thus more weight can be carried) by taking into account headwind, temp and pressure. But....let's keep it simple.

As of now everyone can go.

Boarding completes...the plane pushes back and heads to runway 22. Then it happened. Snow. New performance penalty. A hefty 900 pounds.

New max takeoff weight is 58700. The plane left the gate weighing 59800. The dispatcher reworks the alternate. A new one is found that only requires 1300 pounds of fuel. The flight is legal...as soon as they shed 900 pounds. At this point fuel is the easiest thing to shed.

Offloading fuel is expensive. The fuel can't go back into the ground (for airports with ground refueling) or just any fuel truck. The fuel must be offloaded into a separate container to avoid contamination. Often cheaper to burn it.

Thus the plane will sit somewhere on the airport property with both engines running and the APU burning....to shed that 900 pounds. Taking off and flying lower isn't an option as they are too heavy to takeoff.

If the snow had started before they left the gate they would have likely left behind people or bags.

Believe it or not people are often cheaper than bags. People can be bought off at the gate. Their bags goes on to their destination. If the person goes but the bag stays the airline pays a very hefty fee to have the bag hand delivered to the person.

Hope this answered your question. I truly hate math so I tried to keep it easy. That's all for now...more blogs over the next few days.

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