Narrative:

The MD80 performance manual may have an inaccuracy in the 'typical engine settings in cruise' chart. We set the target N1 setting in-flight at FL330 and found that this N1 setting did not yield sufficient thrust to keep the airspeed from decaying. We may want to consider having someone ride in the jumpseat on a few flts to record engine readings at cruise and compare these data with the chart. If the comparison yields a consistent discrepancy; perhaps we should go back and re-examine the procedure used to generate the N1 data. It seems appropriate to at least spot check the accuracy of the chart in-flight with real airplanes.supplemental information from acn 750989: as soon as I looked over the new 'typical N1 power setting chart;' I knew it wasn't even close to what is the 'typical' N1 settings for an MD80 at cruise. I tried them out on several flts anyway just to see how atypical the settings are. The results were: a decrease of 10 KTS per minute with the engine anti-ice off and decrease of 20 KTS per minute with the engine anti-ice on -- not a very useful chart. There are 3 reasons why this new N1 chart is not accurate. When an MD80 operates in the left side of the chart (the maximum altitude blocks) the N1 gauge reads around 85 percent at the beginning of the cruise; and the pitch attitude is approximately 3 degrees. As you burn off fuel the pitch attitude decreases to about 2 degrees and the N1 decreases to around 82 percent. When these two conditions are met; the aircraft is ready to step climb 2000 ft. The idea that the aircraft will be in a maximum altitude block and stay in the air with the N1's set at 82 percent is wishful thinking. For an MD80; there are only two primary instruments that are necessary for aircraft control -- the attitude indicator (horizon; aircraft symbol; and sky pointer) and the N1 gauge. For cruise climbs the pitch attitude starts at 8 degrees at 10000 ft and decreases 1 degree every 2500 ft until the aircraft passes 23000 ft where the pitch attitude remains at 3 degrees up to the service ceiling of the aircraft. Any momentary variations in this pitch schedule is due to the gain or loss of kinetic energy due to a changing wind vector. At the temperature variation that we spend most of the year flying in; the N1 setting for climb is 91 percent once you leave the inversion layer and it increases about 1 percent every 10000 ft up to 94 percent around 35000 ft. During the climb; if you level off below your maximum altitude; you will have to decrease both pitch and power to maintain a constant airspeed. When you level off at your maximum altitude the pitch stays at 3 degrees and only power is reduced. The marked decrease in vertical climb rate during the last 1500 ft of climb to the maximum cruise altitude is an indication that the aircraft is reaching its maximum altitude (because both the wing and engines are reaching their performance limits). With no change in wind vector and a constant 3 degree pitch; a decrease in climb rate to around 600 to 800 FPM with the N1 set at climb power is reason to stop the climb even if chart says you can go higher -- you may be heavier than you think. At cruise; if you are flying an MD80 at normal cruise speeds with a pitch attitude of 3 degrees; the wing doesn't have any more angle of attack left in it to climb with. If the N1 gauges read 85 percent; the climb is more than likely engine limited. It also means that if you encounter turbulence or have to turn on the anti-ice; you will also need to select climb power on the tri or your speed will more than likely bleed off. You have to remember; however; that this technique doesn't work at 37000 ft since the difference between cl and cr is only about .01 EPR. Understanding the relationship between pitch attitude; N1 settings; and aircraft altitude (not something the teaches); is of course not a substitute for charted values; however; they do conform to the basic physics of flight and will more often than not tip you off when someone is not right with the aircraft's performance during climb and cruise. The purpose of this new chart is of course to help keep pilots from falling out of the sky. It sure wouldn't be helpful in that regard. I've been on the MD80 a long time and my personal belief is that MD80's began falling out of the sky about the time we got GPS and a map mode in the cockpit. After that point; pilots didnT have to be constantly watching the instrument panel. The thing about the MD80 is that the automation is not at the level of reliability of what you would expect from a part 121 aircraft in this day and age. It also has the habit of not telling you about it when it does malfunction. It will sometimes surprise even the most vigilant pilot. The only practical way I think this problem of MD80's getting too slow at altitude will be solved is to put an angle of attack indicator in the cockpit with an alert to warn when the aoa reaches a preset value. It would be a cheaper fix than cleaning up after one of our flts gets into an attitude it can't recover from.callback conversation with reporter acn 750988 revealed the following information: the first officer reports the intention of this type of chart is to protect against low speed flight in the event of other malfunctions. In this case; use of these charts would promote slow airspeed decay at a rate that would be undetected by the flight crew unless they were looking at the airspeed indicator. Reporter wonders if the charts were generated by historical data or computer generated. Reporter stated in any case; suggested power settings do not apply to current MD80's whether because of computational error or aging aircraft.

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Original NASA ASRS Text

Title: AN MD80 CREW REPORTS AIR SPEED DECAY RESULTS WHEN PERFORMANCE CHARTS ARE USED TO SET CRUISE POWER SETTINGS.

Narrative: THE MD80 PERFORMANCE MANUAL MAY HAVE AN INACCURACY IN THE 'TYPICAL ENG SETTINGS IN CRUISE' CHART. WE SET THE TARGET N1 SETTING INFLT AT FL330 AND FOUND THAT THIS N1 SETTING DID NOT YIELD SUFFICIENT THRUST TO KEEP THE AIRSPD FROM DECAYING. WE MAY WANT TO CONSIDER HAVING SOMEONE RIDE IN THE JUMPSEAT ON A FEW FLTS TO RECORD ENG READINGS AT CRUISE AND COMPARE THESE DATA WITH THE CHART. IF THE COMPARISON YIELDS A CONSISTENT DISCREPANCY; PERHAPS WE SHOULD GO BACK AND RE-EXAMINE THE PROC USED TO GENERATE THE N1 DATA. IT SEEMS APPROPRIATE TO AT LEAST SPOT CHECK THE ACCURACY OF THE CHART INFLT WITH REAL AIRPLANES.SUPPLEMENTAL INFO FROM ACN 750989: AS SOON AS I LOOKED OVER THE NEW 'TYPICAL N1 POWER SETTING CHART;' I KNEW IT WASN'T EVEN CLOSE TO WHAT IS THE 'TYPICAL' N1 SETTINGS FOR AN MD80 AT CRUISE. I TRIED THEM OUT ON SEVERAL FLTS ANYWAY JUST TO SEE HOW ATYPICAL THE SETTINGS ARE. THE RESULTS WERE: A DECREASE OF 10 KTS PER MINUTE WITH THE ENG ANTI-ICE OFF AND DECREASE OF 20 KTS PER MINUTE WITH THE ENG ANTI-ICE ON -- NOT A VERY USEFUL CHART. THERE ARE 3 REASONS WHY THIS NEW N1 CHART IS NOT ACCURATE. WHEN AN MD80 OPERATES IN THE LEFT SIDE OF THE CHART (THE MAX ALT BLOCKS) THE N1 GAUGE READS AROUND 85 PERCENT AT THE BEGINNING OF THE CRUISE; AND THE PITCH ATTITUDE IS APPROX 3 DEGS. AS YOU BURN OFF FUEL THE PITCH ATTITUDE DECREASES TO ABOUT 2 DEGS AND THE N1 DECREASES TO AROUND 82 PERCENT. WHEN THESE TWO CONDITIONS ARE MET; THE ACFT IS READY TO STEP CLB 2000 FT. THE IDEA THAT THE ACFT WILL BE IN A MAX ALT BLOCK AND STAY IN THE AIR WITH THE N1'S SET AT 82 PERCENT IS WISHFUL THINKING. FOR AN MD80; THERE ARE ONLY TWO PRIMARY INSTRUMENTS THAT ARE NECESSARY FOR ACFT CTL -- THE ATTITUDE INDICATOR (HORIZON; ACFT SYMBOL; AND SKY POINTER) AND THE N1 GAUGE. FOR CRUISE CLBS THE PITCH ATTITUDE STARTS AT 8 DEGS AT 10000 FT AND DECREASES 1 DEG EVERY 2500 FT UNTIL THE ACFT PASSES 23000 FT WHERE THE PITCH ATTITUDE REMAINS AT 3 DEGS UP TO THE SVC CEILING OF THE ACFT. ANY MOMENTARY VARIATIONS IN THIS PITCH SCHEDULE IS DUE TO THE GAIN OR LOSS OF KINETIC ENERGY DUE TO A CHANGING WIND VECTOR. AT THE TEMP VARIATION THAT WE SPEND MOST OF THE YEAR FLYING IN; THE N1 SETTING FOR CLB IS 91 PERCENT ONCE YOU LEAVE THE INVERSION LAYER AND IT INCREASES ABOUT 1 PERCENT EVERY 10000 FT UP TO 94 PERCENT AROUND 35000 FT. DURING THE CLB; IF YOU LEVEL OFF BELOW YOUR MAX ALT; YOU WILL HAVE TO DECREASE BOTH PITCH AND POWER TO MAINTAIN A CONSTANT AIRSPD. WHEN YOU LEVEL OFF AT YOUR MAX ALT THE PITCH STAYS AT 3 DEGS AND ONLY POWER IS REDUCED. THE MARKED DECREASE IN VERT CLB RATE DURING THE LAST 1500 FT OF CLB TO THE MAX CRUISE ALT IS AN INDICATION THAT THE ACFT IS REACHING ITS MAX ALT (BECAUSE BOTH THE WING AND ENGS ARE REACHING THEIR PERFORMANCE LIMITS). WITH NO CHANGE IN WIND VECTOR AND A CONSTANT 3 DEG PITCH; A DECREASE IN CLB RATE TO AROUND 600 TO 800 FPM WITH THE N1 SET AT CLB POWER IS REASON TO STOP THE CLB EVEN IF CHART SAYS YOU CAN GO HIGHER -- YOU MAY BE HEAVIER THAN YOU THINK. AT CRUISE; IF YOU ARE FLYING AN MD80 AT NORMAL CRUISE SPDS WITH A PITCH ATTITUDE OF 3 DEGS; THE WING DOESN'T HAVE ANY MORE ANGLE OF ATTACK LEFT IN IT TO CLB WITH. IF THE N1 GAUGES READ 85 PERCENT; THE CLB IS MORE THAN LIKELY ENG LIMITED. IT ALSO MEANS THAT IF YOU ENCOUNTER TURB OR HAVE TO TURN ON THE ANTI-ICE; YOU WILL ALSO NEED TO SELECT CLB POWER ON THE TRI OR YOUR SPD WILL MORE THAN LIKELY BLEED OFF. YOU HAVE TO REMEMBER; HOWEVER; THAT THIS TECHNIQUE DOESN'T WORK AT 37000 FT SINCE THE DIFFERENCE BETWEEN CL AND CR IS ONLY ABOUT .01 EPR. UNDERSTANDING THE RELATIONSHIP BETWEEN PITCH ATTITUDE; N1 SETTINGS; AND ACFT ALT (NOT SOMETHING THE TEACHES); IS OF COURSE NOT A SUBSTITUTE FOR CHARTED VALUES; HOWEVER; THEY DO CONFORM TO THE BASIC PHYSICS OF FLT AND WILL MORE OFTEN THAN NOT TIP YOU OFF WHEN SOMEONE IS NOT RIGHT WITH THE ACFT'S PERFORMANCE DURING CLB AND CRUISE. THE PURPOSE OF THIS NEW CHART IS OF COURSE TO HELP KEEP PLTS FROM FALLING OUT OF THE SKY. IT SURE WOULDN'T BE HELPFUL IN THAT REGARD. I'VE BEEN ON THE MD80 A LONG TIME AND MY PERSONAL BELIEF IS THAT MD80'S BEGAN FALLING OUT OF THE SKY ABOUT THE TIME WE GOT GPS AND A MAP MODE IN THE COCKPIT. AFTER THAT POINT; PLTS DIDNT HAVE TO BE CONSTANTLY WATCHING THE INSTRUMENT PANEL. THE THING ABOUT THE MD80 IS THAT THE AUTOMATION IS NOT AT THE LEVEL OF RELIABILITY OF WHAT YOU WOULD EXPECT FROM A PART 121 ACFT IN THIS DAY AND AGE. IT ALSO HAS THE HABIT OF NOT TELLING YOU ABOUT IT WHEN IT DOES MALFUNCTION. IT WILL SOMETIMES SURPRISE EVEN THE MOST VIGILANT PLT. THE ONLY PRACTICAL WAY I THINK THIS PROBLEM OF MD80'S GETTING TOO SLOW AT ALT WILL BE SOLVED IS TO PUT AN ANGLE OF ATTACK INDICATOR IN THE COCKPIT WITH AN ALERT TO WARN WHEN THE AOA REACHES A PRESET VALUE. IT WOULD BE A CHEAPER FIX THAN CLEANING UP AFTER ONE OF OUR FLTS GETS INTO AN ATTITUDE IT CAN'T RECOVER FROM.CALLBACK CONVERSATION WITH RPTR ACN 750988 REVEALED THE FOLLOWING INFO: THE FO REPORTS THE INTENTION OF THIS TYPE OF CHART IS TO PROTECT AGAINST LOW SPEED FLT IN THE EVENT OF OTHER MALFUNCTIONS. IN THIS CASE; USE OF THESE CHARTS WOULD PROMOTE SLOW AIRSPEED DECAY AT A RATE THAT WOULD BE UNDETECTED BY THE FLT CREW UNLESS THEY WERE LOOKING AT THE AIRSPEED INDICATOR. REPORTER WONDERS IF THE CHARTS WERE GENERATED BY HISTORICAL DATA OR COMPUTER GENERATED. RPTR STATED IN ANY CASE; SUGGESTED POWER SETTINGS DO NOT APPLY TO CURRENT MD80'S WHETHER BECAUSE OF COMPUTATIONAL ERROR OR AGING ACFT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.