Flight Operations Report: MD-80 High Altitude Cruise
(revised 2/28/06)
“Maintain thine airspeed, lest the ground rise up and smite thee.” -- old axiom
We are all concerned about the small, but persistent incidence of our fleet's crews
losing flying speed at altitude. Certainly there is a degree of flight crew inattention
involved, but I think there are systemic reasons why this has proven to be such a
difficult problem at our airline. It will continue to be problematic until we (1) recognize
the underlying technical issues, and (2) implement procedures to address them. Recent
changes, such as the implementation of "Max Range Cruise", have only exacerbated
the problem. ASAP reports indicate that high altitude incidents, while precipitated by a
variety of events, are all caused by the same basic set of airplane characteristics.
I’d like to address some things I believe are important and have not been adequately
accounted for in our procedures. I urge the fleet management to approach this issue
with an open mind. Please bear with me if I’m stating the obvious in this report, but it is
important to lay out all the factors in a comprehensive way.
I believe there are reasons unique to the MD80 that expose our crews to a much
greater risk of high altitude incidents. In contrast to the Boeing fleets, the MD-80 has a
rather small wing of an old design that does not perform well at altitude. The small wing
area results in a very high clean wing loading – over 130 lbs/sq ft at high gross weights.
It has a relatively poor Reynolds number and the drag rise at high altitude is steeper
than the modern airfoils on our Boeing airplanes. Compounding the problem is the rapid
drag rise due to transonic shock waves at M.78 and greater, resulting in a fairly narrow
useable speed range at maximum cruise altitude.
The biggest challenge is the high wing loading drives all the operating speed margins
up. Normal cruise mach at high altitude produces an indicated speed that is within 1015 knots of VLD MAX (speed for the best lift/drag ratio). Consequently the MD80 ends up
cruising at indicated speeds that are much closer to VLD MAX than our Boeing fleets.
This is a very unique and significant characteristic of the MD-80 that AA has not
addressed in any of the training or procedures changes to date. Under these conditions
it doesn’t take much of a speed loss for the airplane to slip into a regime where drag is
increasing and acceleration or even maintaining altitude can become difficult or
impossible with the available thrust.
Recent changes to the operating environment may be contributing to the rise of high
altitude incidents in the fleet:
 RVSM procedures result in a higher percentage of cruise operations at or near
maximum altitude capability.
 Introduction of slower cruise speeds give the crews less time to recognize and
respond to speed deterioration at altitude (indicated speed for Max Range Cruise is
very nearly the same as VLD MAX at the limit of the altitude envelope)
Other aggravating factors include cruising at maximum altitude using airfoil and engine
anti-ice, and use of PERF mode to control cruise speed in unstable air at the limit of the
altitude envelope.
Discussion of MD80 high altitude cruise envelope
Following traditional industry practice, we use a 1.3G Buffet Boundary chart to limit
cruise altitude and Mach. Compliance with the charted values ensures a 30% margin is
maintained from that level-flight load which would result in the onset of either low-speed
(stall) buffet or high-speed (Mach) buffet. The 30% margin is somewhat arbitrary, but is
intended preclude the onset of buffet induced either by normal maneuvering or gust
loading.
Understanding the history is helpful. In the early years the industry started out with a
1.1G buffet boundary for cruise in an attempt to maximize range and optimize the thirsty
turbojet engines. Early jet transports like the B-707 cruised at a speed that was typically
centered in the buffet envelope, and as a result some severe turbulence encounters
induced simultaneous low and high-speed buffet. Recovery from this condition requires
aggressive unloading of the wing and immediate descent, and at least two aircraft were
lost in upsets. The required buffet boundary was increased to 1.3G’s This resulted in
lower max cruise altitudes, but it widened the useable envelope and virtually eliminated
buffet-induced upsets. Today’s conventional 1.3G buffet boundary is generous from the
standpoint of preventing buffet-induced upsets. It easily permits 30-deg banked turns,
and even brief encounters with moderate turbulence (the NWS-defined moderate gust
of 37 FPS induces a 1.33G load) without any danger of upset. We need not be
concerned with the adequacy of the buffet boundary.
Most modern long-range wing designs, like the 777, have a high critical Mach number
for best range and altitude capability, and the normal cruise speeds are not as easily
limited by high-speed buffet boundary. However, the MD-80’s normal cruise speed is
well into the high-speed end of the buffet envelope, due to the rather low critical machnumber of the wing. Accordingly MD-80 altitude capability is actually limited in all
circumstances by high-speed - or more accurately - Mach buffet boundary and never
low-speed (stall) buffet. However, simply observing buffet boundary limits provides
no assurance that performance is adequate to avoid slow-speed incidents at high
altitudes.
We generally cruise at mach values of M.78 or less for efficiency, not safety reasons.
At M.76 areas of the MD-80, including upper wing surfaces and engine pylons,
experience some transonic airflow, but shock waves are still very small and
disorganized, permitting cruise at M.76 – .77 without a drag penalty. However, around
M.78 shock waves become well-organized, and drag begins to rise rapidly. At M.81 a
1.0G-loaded wing experiences the onset of buffet, and both drag and buffet become
very pronounced by M.82. This all occurs well before the M.84 MMO is reached.
However pronounced, Mach buffet on the MD80 does not introduce adverse handling or
flight characteristics. Furthermore, above M.82 the drag rise is so steep that
acceleration of even a lightly-loaded aircraft beyond MMO is not even possible in level
flight with available thrust. Therefore, any practical safety considerations with high-Mach
number cruise are minimal; and momentary overspeed in any high-Mach cruise tends to
be self-correcting. If safety is the only concern at high altitude, faster is always better.
Accordingly, the real safety issue for the MD-80 is not either high or low-speed buffet –
it is low-speed drag. It is drag and available performance to which we need to direct
our attention, and for this the 1.3G chart is of absolutely no use. The buffet-boundary
concept we are using to limit cruise altitude and speed does not address the real safety
issues involved in operating at these altitudes. In fact, there is a perverse aspect to the
compliance with the 1.3G chart that dictates that a heavy aircraft be flown at a lesser
Mach value, and hence, slower indicated speed.
Providing a 1.3G buffet chart might tell a crew whether or not the airplane is light
enough to use a given altitude, but it does not tell them what they must do to operate
safely when they get there. For this they need two additional, critical bits of
information:


Minimum safe indicated speed
Normal cruise N1 value
Our crews must be continuously aware of the minimum safe IAS at altitude, and must
ensure the aircraft is never permitted to slow below this speed, irrespective of Mach.
And knowing the normal N1 values for cruise (nominal 83%) will provide a cross-check
against EPR anomalies, which generally result in autothrottle retard.
The aerodynamics of high-altitude flight
Cruise flight below maximum habitable altitudes do not present the same considerations
as cruise in the highest part of the envelope, and aren’t the subject of this discussion. It
is in the upper area of the cruise envelope where problems are going to be encountered
if all of the technical characteristics of the aircraft are not adequately accounted for. But
it is important for efficiency and passenger comfort that our crews be comfortable using
the full altitude capability of the airplane, and that they operate safely while doing so.
Early in the advent of jet aircraft aerodynamicists began to recognize an aerodynamic
phenomena that occurs at high altitudes due to the momentum characteristics of air
molecules. Referred to as “Reynolds Effect” by most aerodynamic texts, it causes
airflow to detach from an airfoil more readily as altitudes above 25,000 ft. Accordingly,
airflow begins to detach at progressively lower alpha values as altitude increases and
higher speeds are required to keep the airflow laminar. This drives all the speeds on
the L/D curve, including VS, and VLD MAX, to higher values at the upper end of the altitude
envelope. This effect can be seen by comparing holding speeds at high and low
altitudes and is the principle reason that published clean minimum maneuver speeds
are not high enough, even at 1.5 VS, to protect the aircraft at high altitudes. Also – and
this is significant -- Reynolds Effect also increases drag for any given alpha, degrading
high-altitude acceleration.
We operate an MD80 fleet of relatively old, high-cycle aircraft. Over time the affects of
dents, patches, pressurization leaks, hard landings, flight control rigging, and other
airframe deterioration take their toll on aerodynamic efficiency. The resulting laminar
flow losses are the most pronounced in high altitude cruise where the airplane is
operating at high alpha, close to VLD MAX, and Reynolds Effect is most pronounced. It is
reasonable to assume these losses to be on the order of 3% or so on average; they will
have a cumulative affect on high-altitude aerodynamic performance by increasing both
drag and reducing still further the max alpha at which laminar flow can be maintained.
It is hard to accurately determine what affect this might have on the stall speed for a
typical aircraft in the fleet, but it only takes a very small reduction in alpha to increase
stall speed 4 or 5 knots, and the attendant effect on VLD MAX is compounded. The PFPM
standards established by the manufacturer permit a variance of 4 knots on a clean,
high-altitude stall break for a new aircraft, and are certainly more than that for older
airplanes in service. The variance in real VLD MAX could be significant – perhaps on the
order of 10 knots or more for a typical aircraft.
This all becomes quite important, given the way we should be operating for best
economy. Our crews need to be willing to use the full altitude capability, climbing to the
maximum altitude allowed by the 1.3G chart, and continuing to step-climb as weight is
reduced. Under ideal circumstances the airplane will be at or near its maximum cruise
altitude for most of the flight, and at the speed that will produce the best range in NAM
(lowest trip burn). All these factors mean that we will be operating at relatively slow
indicated speeds and at or near actual VLD MAX. It is imperative that our crews be made
aware that the real safety issue in maximum-altitude cruise is minimum IAS -- not Mach
number.
Finally, the MD-80 cruise altitude is not technically limited by thrust anywhere in the
envelope (with anti-icing off), nonetheless the amount of excess thrust available to climb
and accelerate is more limited than that of a modern Boeing aircraft at typical cruise
altitudes. When in doubt, cruise speed should always default to a higher, rather than
lower value.
Max Range Cruise procedures
The flight department has recently adopted a “max range cruise” strategy to replace the
industry-standard “99% LRC” approach. While this new strategy might appear to offer
cost savings on paper, I suspect that in reality it might increase actual trip fuel
consumption over the use of conventional LRC. It is not my purpose here to fully
examine the rationale for and feasibility of MRC; rather, I want to point out how the use
of this procedure at maximum cruise altitude can lead to an undesired deceleration to
an unsafe speed.
The industry standard 99% LRC concept is grounded in a number of practical
considerations, among them cruise stability, airframe deterioration, and the
characteristics of the drag curve. It ensures, among other things, that variations in
cruise speed (+- 1%) will not result in any operation at less than VLD MAX.
A 20-year old airplane is not going to have the same drag characteristics as a new
airplane. Moreover the combination of MRC speed at maximum habitable altitude is
bound to result in frequent speed excursions into the low-speed region of increasing
drag. Use of PERF only exacerbates the potential due to the PMS’ intentional tolerance
for speed variations. Under these circumstances, the airplane is only one small
additional factor -- a long-moment, deep atmospheric wave, EPR anomaly, or
autothrottle malfunction – away from a high-altitude episode. The combination places
the crew in an untenable situation, where continuous active monitoring of airspeed is
the only thing that can prevent an inevitable critical performance event.
It is not possible to easily compare the max range cruise IAS to V LD MAX for the weight
and conditions using AA data. However, a cursory check of cruise indications in actual
flight reveals that the MRC speed is only about 5 knots or so greater than the published
holding speed when operating at max altitude. Use of MRC at altitudes 4000 ft or more
below maximum do not result in operation at the low indicated speeds associated with
maximum altitude, and might still be acceptable. It is the combination of MRC and
maximum altitude that creates the potential for an uncontrollable loss of speed.
Evaluation of MRC in line aircraft
I evaluated the actual MRC performance of 22 different aircraft at maximum altitude, in
stable air, during the past few weeks by manually setting the charted fuel flow and EPR
for conditions (autothrottle OFF) and observing the airplane’s behavior. I used digital
fuel flow displayed on the PMS status/input page, corrected for temperature. Averaged
fuel flow generally yielded an EPR setting very close to charted values; in other words,
the EPR and fuel flow seemed to generally confirm that thrust levels were set as
charted. Here are the summarized results.

Not a single aircraft I checked would maintain stable cruise speed at MRC using
the charted values. All of them decelerated below MRC speed, some of them at
alarming rates. Two of the aircraft did stabilize at a speed slightly less than
MRC, both of them at heavier weights and lower altitudes

After allowing the speed to drop as low as was prudent (not below holding
speed), CL was selected and the aircraft accelerated back to a speed slightly
above MRC. Acceleration was painfully slow for all aircraft – on the order of 1
knot per 10 seconds or so.

An attempt was made to find the thrust required to actually maintain MRC. In
every case the FF and EPR required were higher than that charted, in some
cases equivalent to the charted values for much higher speeds – M.75 or M.76

In most instances the airplanes would maintain M.75 or M.76 with thrust set at or
very closed to charted FF and EPR, even though none would maintain MRC with
charted thrust.

Some aircraft seemed to exhibit inherent speed instability in the MRC range,
but not at higher speeds. In other words, it was impossible to find any thrust
setting that permitted stable operation at MRC; the aircraft required continuous
thrust adjustments to maintain speed.
I will acknowledge that it is very difficult to control for all variables in an informal
evaluation of this nature. Atmospheric conditions and accuracy of instrumentation can
contaminate the results. However, I note the relative consistency of results during MRC
at maximum altitude, even with the small sample number of aircraft. I think I have
enough evidence to safely conclude that the typical line airplane is not capable of stable
cruise flight at MRC at maximum altitude using charted thrust settings.
In an old (but still useful) fuel conservation guide for the DC-9, the manufacturer points
out that the range penalty for cruising 1% faster than actual LRC is only half that of
cruising 1% slower; accordingly, using a speed that equates to 99% LRC – 1% fast –
will provide the best overall economy in actual operations. I think this rationale is still
valid; even with modern autothrottles it is impossible to maintain a precise cruise speed.
In fact the PMS by design avoids aggressive corrections to cruise speed and altitude.
By cruising at “max range cruise” speed we are actually spending some time in the
region where range is reduced. I doubt that MRC reduces actual trip consumption – and
there is no realistic way to even independently assess the effects of MRC
implementation.
I would urge the company to re-instate industry-standard 99% LRC cruise procedures
on this fleet.
Use of anti-icing at high altitudes
It could be that the flight operations culture at AA which encourages extensive use of
airfoil and eng anti-ice, even in cruise, could be contributing to the high-altitude
performance issues by stealing most or all of the excess thrust.
There is an urban legend in circulation that failure to use airfoil anti-icing can cause the
loss of both air conditioning packs; as far as I can determine this is a myth that has no
basis in fact or rational technical explanation. I have also been told in training that one
must never use engine anti-ice without also using airfoil anti-ice. Neither of these
superstitions are supported by fact or cited anywhere in AA’s manuals. In fact, there
are times when it might be prudent to use engine anti-ice alone, even though technical
“icing conditions” are not present.
It is well-established fact that airfoil and engine icing as defined by regulation are
virtually non-existent in stable air at high altitudes due to the very small amounts of
moisture that stable, cold air is capable of suspending. The small amount of precipitate
that is present is nearly always in the form of small ice crystals, which cannot form ice
on an airframe in a conventional sense, in the same way supercooled water droplets
found at lower altitudes can do. While it is impossible to say with certainty that there is
a certain altitude above or temperature below where airframe icing will “never” be
encountered, nonetheless it becomes extremely unlikely when SAT is -25 degrees or
less. This is formally acknowledged in many operating manuals, and while it is not in
the MD-80’s AFM, it is still a fact.
It is my understanding from previous contact with the manufacturer’s technical
representatives the airfoil anti-ice system was not designed nor intended to be used for
extended operation at high altitudes. One reason is that the 13 th-stage augmentation
rationale inverts at an altitude of about 33,000 ft due to the mass flow characteristics of
the JT8D (the governing rationale assumes the demand for pressure is always less than
temperature, which is only true at lower altitudes). This characteristic caused a number
of air conditioning and airfoil HIGH TEMP lights early in the service history of the
airplane and can still result in anomalous annunciations today.
Selection of airfoil and/or engine anti-ice at or near the maximum altitudes can reduce
the amount of excess thrust available to the point where it is impossible to maintain
cruise speed. This results in the throttles being commanded continuously to EPR LIM.
This is not a stable cruise situation, and must not be tolerated. Except under very
unusual circumstances continuous use of anti-ice in high altitude cruise is not required
by safety, regulation, type design, or certificate limitations.
The principal reference for the anti-ice requirements is the FAA Approved Flight Manual
(AFM). The Certificate Limitations section of the AFM contains statements that pertain
to some anti-ice systems, such as probe heat, but none requiring use of either engine or
airfoil anti-ice. From this we can determine that the certification of the airplane is not
dependent on any particular pilot operation of anti-ice. The “Normal Procedures” section
does contain the following pertinent and general statements: “Airfoil anti-ice shall be
used in-flight whenever icing conditions exist or are anticipated” and “Engine anti-ice
shall be used in-flight or on the ground whenever icing conditions exist or are
anticipated, and on the ground under the following circumstances….”
The broad definition of “Icing conditions” on page 26 of the DC-9 OM Limitations section
is not found in the FAA-approved AFM Limitations section. As a general statement
regarding icing it is far too limiting and should be removed entirely. The definition of
icing conditions should be addressed in the operating procedures section of the manual
(General, Cold Weather Operations) as it is in the AFM.
The thrust and fuel flow penalties associated with operation of thermal anti-ice at
altitude are substantial – on the order of 10%. With fuel prices as high as they are we no
longer have the option of just publishing unnecessary limitations because they are
comfortably conservative. And with airplanes falling out of the sky at altitude, safety
dictates that we remove impediments to crews that might need all available thrust in
day-in, day-out operations.
Recommended changes to operating procedures and practices
The most important thing we can do to minimize the possibility of more high-altitude
incidents is to create and implement a concept of minimum cruise speed. Simple
awareness of an IAS, below which you never want to go, will circumvent all of the
peripheral issues and confront the basic problem head-on.
The onset of winter flying brings colder temperatures at altitude; this means lower IAS
values for any given scheduled Mach number. It is important to act right away.
I’ve listed the specific actions I think we need to take in order to prevent further highaltitude episodes, preserve the efficiency of the operation, and provide crews with the
information they need to operate safely in the upper end of the envelope.
1.
Procedure: both pilots will bug the minimum cruise speed prior to reaching
cruise altitude, and must reset the bugs at each step-climb. Minimum speed
in cruise is defined as gross weight in thousands plus 120 (e.g., at 140,000lb
minimum cruise speed is 260 knots.) Adding 120 to the gross weight will
provide a minimum speed which is at least 5 knots greater than the highest
actual VLD MAX . Using this procedure in a typical maximum altitude cruise
situation for 140,000 lbs, FL 330, the bug will be set to 260 knots. Cruise IAS
is 265 knots at M.75.
As simple as this is to do, this might be the most important procedure we can
implement to eliminate high-altitude incidents, for a number of reasons.
 The simple action of setting the bug focuses the crew’s attention on the
issue.
 The visual impact of the airspeed pointer across from the bug reinforces
the concern, and provides the crew with an instantaneous, continuous
visual reference to minimum safe cruise speed.
2.
Procedure: IAS in cruise must not be permitted to go below min cruise
speed; if min speed is greater than the target cruise Mach, fly min speed. If
stable cruise cannot be maintained at or above min speed, request and
descend to a lower altitude. If PERF will not maintain a stable IAS at or above
minimum speed, use MACH SEL and ALT HLD modes instead.
3.
Procedure: If it becomes necessary to use either engine or airfoil anti-ice in
cruise, the TRI must be selected to CL (or MCT as appropriate). If the
throttles are continuously commanded to EPR LIM with anti-ice in use, select
a lower cruise altitude.
4.
Operating practice: airfoil and or engine anti-ice need only be used if “icing
conditions exist or are anticipated”, in accordance with the FAA-approved
AFM. It is presumed that the very cold temperatures and low moisture
content of the atmosphere associated with high altitude cruise does not
contribute to the presence of icing conditions.
5.
Prolonged flight in high altitude stratus or cirrus clouds can cause the EPR
probes on the engines to slowly accumulate small ice crystals present in the
air. This is frozen precipitate that does NOT meet the technical definition of
“icing”, but it can, over a long period of time, cause a gradual reduction in the
measured (displayed) EPR ratio and subsequent gradual throttle retardation.
To guard against this subtle thrust reduction, engine anti-ice is recommended
either intermittently for short intervals, or continuously at the discretion of the
crew.
Eliminating the unnecessary use of anti-ice at cruise will not only provide enhanced
speed stability and safer margins, it will also reduce costs.
Final Conclusions
This approach to the problem emphasizes basic airmanship as opposed to rules or
button-pushing as a solution. It is time to nurture a holistic and intuitive understanding of
the cruise characteristics of our airplane in our pilots. The JT8D is an inefficient engine
in comparison to the rest of the fleet, and we need to cruise as high as practicable if we
are to maximize the efficiency of the company’s largest fleet.
Reaching cruise altitude, minimum speed should be bugged and the airplane should be
trimmed carefully. Both pilots need to closely monitor cruise performance for the first 10
minutes or so, paying close attention to speed stability, general autothrottle response,
average thrust versus scheduled EPR, thrust versus EPR limit, and atmospheric
conditions. If stable cruise cannot be achieved at the scheduled altitude or speed due
to icing, mountain wave or other factors, the airplane must be flown at a higher indicated
speed or at a lower altitude. In most cases increasing speed slightly or reducing altitude
by 2000 ft will provide sufficient stability for unusual conditions.
Pilots should be reminded that the JT8D is limited by rotor speed and EGT, not EPR. It
is impossible to damage the engine by making a thrust demand or throttle movement
provided RPM and EGT limits are not exceeded. There is in reality, no way to
“overboost” a JT8D in the same way one might overboost a turbo-charged reciprocating
engine; EPR indications are provided principally for validation of takeoff thrust and
attendant engine-out performance, and secondarily for reasons of economic warranties.
EPR indications at high altitude are subject to numerous errors and are often
inaccurate. This is an RPM -governed engine and that must always be kept in mind; in
a difficult situation at high altitude one should use whatever thrust is available,
irrespective of EPR indications, provided RPM and EGT limits are respected.
There are rumors afloat that the company and/or the FAA might be considering some
sort of punitive approach to the problem, such as lifting the ASAP immunity for selfreporting these incidents. I would strongly recommend against this approach, as
frustrating as it can be to see the high altitude stalls continue. It is possible that without
immunity crews may do things to cover up an incident, such as unreported overtemping
of engines or turning transponders off to mask an unauthorized descent. And, of course,
the specter of a true high-altitude upset looms large. The whole purpose of reporting
programs is to gain important knowledge regarding real operations -- and without
immunity it is impossible to obtain the truth. Let’s keep the guns aimed at the problem,
not the people.
Respectfully submitted:
Jim Hubbard
S80 First Officer (former Check Airman)
6 December 2005