User Guide

Flight Instruction
257
Flight Instruction
256
This leads to an interesting discussion. (OK, maybe it’s boring
- but if you unwittingly violate its rules, you may find the results fas-
cinating! ) We’re dealing with two limiting airspeeds up here, and
they’re getting closer and closer together. Remember, as we climb
higher and higher, the speed of sound (M 1.0) gets lower and lower
(as does our limiting IMN of 0.8). This is reflected by the steady
downward sweep of the airspeed-limit “barber pole.”
At the same time, as we maintain roughly the same true air-
speed, our indicated airspeed is decreasing steadily. Let’s look at
the situation at an altitude of 41,000 feet. Assume we’re cruising
at M0.67.
At high altitudes (with their correspondingly lower air den-
sities), an aircraft must be pulled to a higher angle of attack to
attain a given G-load than in the dense air of lower altitudes. You
don’t normally think of a jet making particularly steep turns at alti-
tude; but, since rate of turn depends on true airspeed, you may
find that it takes a fairly significant bank angle to achieve a nec-
essary rate of turn, for example if you’re making a course change
over a VOR. The common convention among jets is to calculate
the “buffet boundary” - the point at which the airflow begins to
separate from the wings - for a G-load of 1.5, corresponding to a
bank angle of about 45 degrees. Every jet’s performance manual
includes a table called “buffet boundary.” Entering the one for the
Hawker at 41000 feet and, say, 25,000 lbs gross weight, we find
the allowable range between low-speed and high-speed buffet
runs from M0.64 to M0.70. Given standard temperatures, that
corresponds to only about 35 knots.
In other words, the higher we fly, the narrower is the margin
between our maximum and minimum allowable speeds. What
happens if we exceed the maximum? Various things could occur,
including airframe buffet, aileron “buzz”, loss of elevator effec-
tiveness, or a nose-down pitch change or “Mach tuck.” All of
these are caused as shock waves form or move on the airplane. At
the low-speed end, we’re effectively encountering the beginnings
of a high-altitude stall.
CLIMB
Most jets get more and more efficient the faster they go, and
the Hawker is no exception - but it runs up against a few FAA
rules. At most airports, as long as you’re within 2500 feet of the
surface, you shouldn’t go faster than 200 knots. This shouldn’t be
too hard to maintain, particularly while you’re still retracting the
flaps - and by the time a minute or less has gone by, you’ll have
climbed past 2500 feet anyway. The next restriction is 250 knots,
and that one is valid through 10,000 feet.
In the Hawker, with its generous wing area, recommended
enroute climb speeds are relatively low, hovering around 200
knots or a bit less. You’ll probably find it easier to just hold about
a 15 to 20 degree pitch angle, even if that puts you a few knots
above the most efficient speeds at light weights; anything steeper
than 20 degrees feels pretty uncomfortable, particularly for pas-
sengers in aft-facing seats.
As you climb past 35,000 feet, you can improve fuel econo-
my by setting the #1 and #2 MAIN AIR VALVE switches to their cen-
ter LP ON position. (Just don’t forget to set them back to ON before
you reduce power for the descent, or you won’t have enough air-
flow to keep the cabin pressurized). As soon as the fuel level in each
wing tank has decreased below 3300 lbs you should start transfer-
ring fuel forward from the ventral tank; just move the leftmost lever
at the back of the pedestal down to the bottom of its slot. Within a
moment or two, the little indicator between the two main fuel
gauges should change from FULL to a “barber pole” pattern.
Check it from time to time; until it indicates EMPTY, you’re
restricted to a maximum indicated airspeed of 280 knots, and the
airplane must not be landed with fuel in the ventral tank except in
an emergency.
THE DREADED “COFFIN CORNER”
And what’s the maximum allowable speed once you’ve
emptied the ventral tank? Well, at sea level it would be 335 knots;
at altitude, it’s either M 0.80 or its equivalent airspeed. Luckily,
you don’t have to work that out: instead of a single redline, the ASI
tape has a “barber pole” that’s positioned by the air data comput-
er to indicate the maximum allowable speed. You’ll notice that as
you’ve been climbing, the barber pole has been sneaking down
toward your current indicated airspeed.