October 26, 2010

Questions and Answers

I’ve been getting a number of questions from readers lately…some general in nature and some pretty specific.  I thought it might be fun to post some of the questions and my responses to them.  If you would like to submit a question, there’s a button in my profile you can use to send me an email.  If I post your question, all personal and identifying details will be removed.

The following questions are from a guy flying an MD80 simulator on his home pc:


Question: Ground Power (GPU) vs. APU power. Which one do you use more often?  It is my thought that at some point before the flight, you need to use the APU for bleed air supply to start the engines.  Do you just leave the APU on all the time on the ground or do you turn it on just before engine start?

Answer:  Pre-Conditioned Air (PCA) is usually available to heat and cool the aircraft on the ground, so we typically use ground power and PCA until about 15 minutes before departure.  Sometimes PCA is unavailable or the unit does not provide a sufficient amount of air to heat or cool the jet, in which case we leave the APU running the whole time we're on the ground. At about 15 minutes prior to departure, we start the APU to provide the air we need to start the engines.  If the APU is inop, a start cart is used to provide the air we need to start the engines.  The start cart is also capable of providing the air needed to run the air conditioning system if both PCA and the APU are inop or unavailable.  The APU burns approximately 200 pounds of fuel per hour, so it is economical to leave it off as long as possible.


Question:  What kind of reserve fuel do you plan on. When flying in the sim in the MD82 I try to land with no less than 7k lbs of fuel. What do you guys shoot for in the real world?

Answer:  Generally, I like to land with at least 6,000 pounds.  We are regularly planned to arrive with around 5.8 on a good day with no expected delays, but most Captains will rarely accept less than 6.  The legal minimum for an MD80 is about 4.3 (45 minutes of fuel), which in my opinion is nuts.  Our manual does not even allow a go-around with less than 5k, so why on earth would I accept any less.  The problem with accepting less than about 6k is that if something goes wrong at the last minute, you may not have the fuel to divert to even a close airport.  A diversion to an airport less than 20 miles away could easily consume 2000 pounds of fuel.  Also, the flight plan does not consider many of the fuel sucking variables that we encounter every day.  All that said, 6k is about as low as I like to go…thankfully most of the Captains I fly with agree.


Question:  I sometimes use a simulator program called topcat which produces Vspeeds and t/o and landing data for fs. Do you use a real world software program to determine these or do you use paper charts in reference to OAT and weight?

Answer:  We get two things from dispatch before our flight...a Flight Plan and a Departure Plan.  The Flight Plan has all the en-route info we need and the Departure Plan tells us everything we need to takeoff.  The Departure Plan provides settings for flaps, CG, trim, power and V speeds and is good for the planned departure weight plus 2,000 pounds and planned temperature plus 2 degrees.  If we close out 2,001 pounds over the planned weight or if the actual temperature is 3 degrees above or below plan, then we must get new numbers.  After we push away from the gate, we get a “closeout” over the ACARS that provides actual weight, CG and trim settings for takeoff.  If, for some reason, we are required to get new takeoff data or the Departure Plan does not provide data for the runway in use, we can get new data over the ACARS while we taxi to the runway.


Question:  I am kind of confused about the landing speeds. I’ll get a Vapp and a Vref speed and normally shoot to be at Vapp speed at 1000ft then slow to Vref at 300 feet? How do the pros do it?

Answer:  (Thanks for the “pros” comment by the way).  We bug the min maneuver speeds for each flap configuration and then bug approach speed.  Specifically, the top bug is the minimum speed to fly with a clean wing…flaps and slats retracted.  The bottom bug is Vref.  That way, as you begin to slow for the approach, you know as you approach a bug on the airspeed indicator that you need more flaps.  It is always a good idea to extend flaps closer to the min maneuver speed than the max speed for the flap setting to reduce stress on the flaps and the airframe. 


I generally cross the final approach fix between 170 and 180 knots with flaps set to 11 degrees...anything faster than 180 and you will almost surely have trouble being configured in time to be stabilized at 1000 feet.  If you are not stabilized on speed, on glide path with the engines stabilized at approach thrust by 1000 feet above touchdown, a go-around is required.  At about 1800 feet above touchdown elevation, I reduce the throttles to idle, lower the flaps to 15 and drop the gear.  As soon as the gear is down I lower the flaps to 28 then flaps 40 as we slow.  I then push the throttles up to around 1.3 EPR and stabilize at about VREF plus 10 knots.  This all allows me to be fully configured by the 1000 foot requirement.  I then fly VREF plus 10 knots until the flare.


I hope you find this information helpful.  Good luck with your sim.

5 comments:

  1. Your a great writer...Thanks for answering my questions!

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  2. Thanks for sharing your experiences. Love to read it!

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  3. Good stuff, as always!

    Jon

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  4. .81M! Wow! I didn't know an MD-80 could go that fast! Nice blog.

    Ryan

    ReplyDelete