Cap’n Aux Answers Your AvGeek Q’s – Part III

#avgeek #aviation #blog #airline

This week, we get more technical about just how things work in the Airbus cockpit . . . .
Cap’n has the aircraft!
Once again, Dave W. sent in a nice round of technical Q’s, so we’ll start with some of his…
Do you have to use both left / right hands on the control stick, based on where you sit?
In the Airbus, if you’re a First Officer, i.e., flying from the right seat, you fly the joystick with your right hand.  Captains, in the Left Seat, use their left hand.  I always joke that, being left handed, I’m a “Born Captain!”

You may think it would be strange flying with the “wrong hand,” like writing with your opposite hand.  But it takes all of 5 minutes to get used to it!

Turnin’ & burnin’!

What is the fuel burn-rate of your aircraft?  Taxi?  Cruise?  Climb?

You can Google the tech manuals and find all that stuff, so I’ll give you my personal Rules of Thumb, gleaned over my 18 years in Fifi!  

First of all, “miles per gallon” doesn’t work in an airplane.  Fuel is a volatile gas, and temperature and pressure affect the volume.  So, we use weight, in pounds (1 gal. = 6.4-6.8 lbs.)  Secondly, our speed over the ground is dependent on winds, just like a boat in a river is affected by current.  So, we use “pounds per hour” burned.

In short, we burn about 4-6,000 lbs per hour at cruise, depending on altitude and load.  Conservatively, that’s about 100 lbs per minute.  This makes it very easy to determine how many minutes you have for a given fuel load.


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This week, we had to hold inbound to CYYC due to a snowstorm…

The most common time we’ll use this estimate is when we have to enter holding and need to know how long we can hold before having to divert to our alternate airport.

For taxi, we’ll burn between 300-800 lbs, depending on how much traffic, taxi time, etc.

How is your route determined by the dispatcher?  Factors?
By the Dispatcher, via the computer.  The computer will factor in all the data, including predicted winds aloft, weather, and traffic flow, to come up with an ideal route and altitude.  Often we will step up or down in Flight Levels to take advantage of winds as well.
Often, our “best route” is a mighty circuitous one!  That is, if we go the Great Circle Route, i.e., “straight to destination,” we’ll burn MORE gas and take LONGER to get there!

The Loadmaster will use a sophisticated computer program, like this ocean vessel’s, to calculate W&B data.
How is an aircraft balanced, when it is loaded?  What does the Loadmaster do? 
Weight and Balance loading is critical, even in large airliners.  

(Addendum: Bagram B-747 crash speculated to be improperly loaded or shifting cargo.  Link to crash video—Warning, graphic!!:

Like ships on the sea, they have to be balanced properly and not overloaded. We have a “Loadmaster” who collates all the data—passengers, cargo, fuel, etc.  That is then loaded into a sophisticated W&B computer to determine proper loading.
On some military aircraft, especially cargo carriers, the Loadmaster actually flies onboard as part of the crew.

W&B data gets loaded into the MCDU prior to takeoff.

We receive the “Numbers” on taxi out, and have to load the data into the onboard computer so it can properly assess actual weight, fuel burn, etc.  If you’re ever on a plane or listening to Tower ATC on and a pilot says they’re “waiting for the numbers,” that’s what they mean!
Cost Index today: 35!
What is the Cost Index, and how do you use it?
The Cost Index is a pretty cool feature.
First, a quick lesson: the faster a jetliner goes, the more fuel it burns.  And we’re talking hundreds, if not thousands, of gallons!  That’s the reason, when you’re late, the Captain may elect not bump up the speed to make up for it.  Gobs more fuel burned for only a few minutes made up—and he may need to hold that extra fuel in reserve for potential weather deviations, traffic holds, etc.

For each flight, the computer can calculate the ideal climb and cruise speed for the given price of fuel, balanced against aircraft weight, cruise altitude, fuel burn, winds, length of leg, etc. etc.  This number is entered into the MCDU and would dictate all the speeds–climb, cruise, descent, for maximum savings.

Here’s a PDF download from Airbus regarding the Cost Index.  It’s crazy-hi tech, for you AvGeek engineers!

These days, we do a blanket number of “35”, which gives a decent average fuel savings without going through all the calculation hoops for each flight.  If we are ahead of schedule, we’ll enter a new cost index of “10”, which slows us back.  We still arrive on time or early, but saves gobs more fuel.  The highest number you can enter, which gives the fastest speed (usually M.80 or so), is 99.

I understand that on take off, you advance to 50% till stabilized then set FLEX (Flexible temp, or Reduced Thrust) or TOGA (takeoff/go around thrust) detent.  I seem to remember reading that the engines have a dead zone where you have to power straight through – you ever heard of this or am I miles wide of the mark?
You’re right on the money!  The older model (the A1’s I believe) were found to have a “harmonic zone” where the fan blades would receive excessive fatigue, and wear out early.  Later models addressed this via the ECU (Engine Fuel & Control Unit, which would keep the power out of this zone), but to be safe we were trained to stabilize the power at about 50% N1 for a couple seconds before cobbing it to takeoff power. 


On takeoff, we pause a few seconds for spool-up at the “T” before adding full power.

Coincidentally, the “safe zone” is somewhere around the “T”, as in “Thrust,” on the “A/THR” notation on the levers.  We still use that method today.

Upon rollout after landing how do you deactivate the autobrakes, is it by tapping the toe brakes?
Again, right on the money, my friend!

“TO SHIFT” (artificially highlighted in red) tells the computer how far down from the runway end we are departing. (Note: MCDU=Multipurpose Control Display Unit–our computer interface with the airplane.)Thanks, Rory, for this great pic!
Hey Capn Aux, I’m just wondering what does the “TO SHIFT” on the Take Off page of the MCDU mean, and what is it used for? Thanks.
—Rory F.
Funny you should ask, we just used it! When you throttle up on the runway for takeoff, the MCDU updates our position to the IRS system, knowing we are taking off at the end of, in your case, 25R. If we take off from an intersection, however, we need to tell the box how far away we are from that.  We took off on KJFK Rwy 31L at intersection Kilo-Echo, for example, so we entered “3600”, as in 3,600 feet from the rwy end!
25R Runway end, vs. A3 intersection.  Thanks again, Rory, for the pic!
So, in my example above, if I took of from Intersection A3, I would enter the distance from that point to the RWY25R point?
Correct!  You’ll find the distance data on the Airport Diagram. A3 Intersection takeoff.  Performance must be calculated to ensure a safe takeoff from this intersection, and the MCDU must be told the “TO SHIFT”—ie, distance from the runway end to A3, in order to update the IRS as to the aircraft’s position.

If you forget to plug it in, there will be a bit of a “bias” error built in to the NAV data, but the constant updating from VOR triangulation (& GPS, if equipped) will eliminate the bias error pretty quickly, so it normally wouldn’t be a major issue.

Once you are cleared for a visual approach to a runway, can you still do an ILS?
—Zack S.
Yes.  Think of an ILS as a straight-in approach.  During an actual ILS, you would be vectored to the final, straight in ILS approach somewhere OUTSIDE the FAF (Final Approach Fix, 5-7 miles from the runway.)  The FAF is also about where you will intercept the glideslope of an ILS.  The ILS glidslope is about the same descent path you’d use, regardless of whether on instruments or visual.

Either way, flying visually or doing the official ILS, the easiest thing to do is arm the ILS approach to intercept, and fly the course and track to the runway.  So, while you’re flying visually, you can still “back it up with the ILS.”  It’s just good practice to keep you more aligned for a good landing.

Operationally, being cleared for a visual or an ILS are virtually the same thing.  In either case, ATC is vectoring you to intercept the final approach course on a specific heading, usually within 30 degrees of final.  If you are cleared for a Visual approach, the rules are much more relaxed for aircraft clearance.

Also, is it possible to do a visual with Autopilot? Or only flying by hand?
You can hand fly an ILS or visual approach, and you can fly an ILS or visual approach on the autopilot.  Either way is fine—pilot’s choice!

What may be confusing you is that, once you call the field “in sight,” ATC can clear you for a visual approach w/in 30 miles of the airport.  So, you could be on a downwind, base, whatever, when you are cleared.  So, it’s up to you to turn however you want.  But, eventually, you’re going to end up on Final, on a straight-in approach to the runway, 5 miles or so out.  The ILS localizer course will be exactly on this track as well, so it’s easiest just to fly yourself to an intercept course then arm the localizer/ILS button to intercept.

Otto has the aircraft!
Most pilots “kick off” the autopilot on short final and land by hand.  Otherwise, the autopilot can do an “Autoland.”  But, certain parameters have to be met, regarding operational equipment on the plane and on the ground, for an autoland to take place.
What are the procedures involved in a diversion, specifically how you update the MCDU?  Are you cleared to a waypoint or fix by ATC and then build the plan from that point? Do you delete the existing plan and build a new one?
—Dave W.
We type the Alternate into the INIT page before the flight, to estimate the fuel needed and keep an eye on the extra.
If we actually need to divert, we will “Line Select” a random waypoint from the left side.  On the page that pops up, we can type in a “New Dest,” say, “KDEN” (Denver Int’l.) From there, we manually build the flight plan back in. 
 Most often, ATC will vector us to a waypoint along that route.

The Gimli Glider–now serving downtown Gimli, Manitoba, right from the highway!

What is your typical Fuel On Board on arrival – I’m guessing a couple of thousand kilos? – (or do you work in lbs – remember the Gimli Glider)
Haha thanks for reminding me of the Gimli Glider! (Wiki:

Standard arrival for our fleet of A320-class is around 4,000 lbs, which gives us around an hour of fuel.  Get much below that and we start getting nervous!

Several related Q’s by Dave W. and Junior M. about taxiing and turning: 

How do pilots know how to time turns when taxiing?  Let’s say a pilot is taxiing on to the runway and the pilot gets so far out without turning that all the passengers are able to see the runway straight ahead, how do they know when to make that turn to get ready for takeoff?
—Junior M.
Haha good Q!  Not really sure of an answer, other than you learn to “eyeball” it!  We fly the plane so much, it becomes second nature.  Even so, we all err a little bit here and there, so hopefully we have some fudge factor!

At what sort of speed can you take the hi-speed exit off the runway?
—Dave W.

We are never watching the “speedometer” (airspeed indicator) at that point because it’s irrelevant!  I’m guessing somewhere around 40-50 kts, but we are still romping on the brakes pretty good at that point.  By the time we turn onto the taxiway, we are down to horse n buggy speed (a little aviation trivia: the first Federal Aviation Regulations, circa 1920’s, called for a taxi speed of “no more than a brisk walk”)

I play a lot of flight simulators.  When I’m on the ground taxiing, I have struggle doing turns at 30mph, so I wondered, what is the taxiing speed and is it hard to turn while around 30-40mph?

—Junior M.
Wow, you’re a speedracer!  Our max taxi speed STRAIGHT AHEAD is 30!  To turn, we’re down around 10-15 max!

Reminds me of a fun video (caution, I think they’re cussing in German!)

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If so, I invite you to COMMENTSHARETWEETLIKE, EMAIL &/or +1 below!

It’s right after “Cap’n Aux links and just before the next post.
It looks like this: 
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“Cap’n Aux, have you ever…”

A day in the life of an airline pilot
Technical Questions, the Sequel

Question Potpourri

PART IV, 2.0
Question Potpourri, the Sequel

DON’T FORGET, Weds @ 11:00am PHX:
preempting our Q&A series

Posting April 10 @ 11:00PHX:
Cap’n Aux answers readers’ Q’s—Part III, v. 2.0!

An AvGeek’s technical wet dream continues!
Sample Q’s:
—What are some of the controls in the cockpit, and how do you use them?
—Bill K.
—How do you cold start the A320?
—Dave W.
—How do you memorize the overhead panel?
—Junior M.

Posting April 24 @ 11:00PHX:

Cap’n Aux answers readers’ Q’s—Part IV
YOUR Q’s that you’ve been sending me during this series!