Plane Answers: Minimum fuel requirements and sudden drops inflight

Welcome to Gadling’s feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!

Frank asks:


A couple of questions for you:

  • How much reserve fuel does your plane normally carry? For how many minutes of flight or miles?
  • Who decides how much extra fuel to carry, the captain, FO, company or else?
  • Do you have a way to check how much fuel the plane is consuming? My Altima has a nice gauge that lets me know that, I was wondering what a multi-million dollar aircraft might have?

Thanks Frank.

There’s been some attention in the press lately about some airlines cutting back on the extra fuel carried because of the extra expense in adding unnecessary weight to the airplane.

The FAA requires that domestic airlines carry enough fuel to continue to an alternate airport plus an additional 45 minutes after that. The alternate airport has to have good weather in the forecast.

Our flights have been averaging 70 minutes of additional fuel which works out to almost 500 miles. I can personally think of just a few times when we have been in a holding pattern and needed to divert because we were getting close to our minimum FAA fuel (45 minutes plus enough to get to our alternate airport). In two of those cases, we were allowed to land at our destination. At no time were we sweating the amount of fuel on board.

Before each flight, the captain reviews the fuel requirements, the fuel computed by the dispatcher and the weather at our destination and alternate airports. If he feels we need more, he’ll pick up the phone and ask for an extra one or two thousand pounds from the dispatcher. The fuel decision is up to the dispatcher and the captain, but I’ve never heard of a captain being refused an extra fuel request.

During the flight, we have a fuel log print-out that comes up via ACARS and looks like this:

This fuel plan shows the time and the amount of fuel we are computed to have at each waypoint. It takes into account the forecasted winds, the altitude we’ll be flying at and the weight of our aircraft.

We write down the time and actual fuel as we cross each waypoint. Typically we’ll be up or down a few hundred pounds and a couple of minutes. If there’s a significant difference, we’ll look into the cause and consider diverting if we feel it could be necessary. The company also tracks our fuel burn via automatic updates which are sent from the airplane to ground VHF radio stations and then forwarded to the dispatcher.

As for any fancy indications in the cockpit, we do have a gauge that shows the fuel flow in thousands of pounds for each engine. On a 757, they read around 4,000 lbs an hour for each engine at cruise.

Cassandra asks:

Here’s another question for you (again, hyped by the media!). Why is it they love to report when going through turbulence that the plane dropped 500 feet or 1,000 feet? How do they get this information anyway and come up with these numbers?

Is this even possible?

There is a propensity for passengers and the media to describe a rough flight in terms of how far the aircraft “dropped.” But as you suspected, planes don’t just drop. I’ve used the analogy in the past that it’s like driving in your car and suddenly finding yourself on an interstate two miles away.

The media and a few movies have reinforced this idea that airplanes can hit ‘air pockets’ and drop hundreds or thousands of feet.

As I mentioned in a previous Plane Answers post about turbulence, even during some of the roughest air, we don’t gain or lose altitude generally.

There is one exception, however. If an airliner were to get far too slow, the resulting recovery back to a safe airspeed would require an immediate descent.

And on a similar topic, Kat wonders:

Sometimes when we are cruising in the air, there are moments when its feels like we have dropped a little bit and you get that roller coaster feeling and my stomach moves into my throat a little bit. Is that just the pilot maintaining our altitude? Is it supposed to happen? It’s scary sometimes because it can be a lot or very small–and yes I am one those people who are afraid of flying.

As we get to the higher altitudes, the flight controls are a bit more sensitive. So if a pilot is flying without the autopilot, the movements can be exaggerated, especially for passengers riding in the back. The autopilot can occasionally have some minor oscillations when it’s trying to level off or maintain altitude.

Imagine how smooth a road would have to be to drive a car at 500 to 600 miles per hour. Even the slightest change in the pitch of an aircraft will tend to make your stomach queasy.

Occasionally we’ll also get some oscillations when we approach a turbulent area. As the speed of the airplane increases or decreases when approaching changing weather, the autopilot adjusts to maintain altitude. This causes some very short climbs or descents of just a few feet that you’ll feel in your stomach.

As I’m sure you know, these movements aren’t dangerous for the airplane, but they can certainly be annoying. It’s just another reason everyone should keep their seat belts fastened while seated.

Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he’ll use it for next Monday’s Plane Answers.

Check out his other blog, Cockpit Chronicles to travel along with him at work.