#7a: Manoeuvring at Reduced Airspeed (Speed Variations)

#7a: Manoeuvring at Reduced Airspeed (Speed Variations)

Aim:

To determine your ability to maintain safe flight control in all configurations while manoeuvring at speeds lower than normal cruise.

At an operationally safe altitude you will be asked to reduce airspeed for the purpose of establishing the aircraft in a stabilized, full-flap, final approach speed; once the slower speed is established, the Examiner will ask you to perform manoeuvres in conjunction with the extension and retraction of gear and flaps.1

Assessment is based on the following:

maintain airspeed within ± 10 knots;
maintain heading within ± 10°; and
maintain altitude within ± 100 feet.

Discussion

Speed variations in the Seneca are extremely challenging and a tremendously effective way of getting a good handle for the aircraft, and its stability at various points in the speed range. First of all, start right away in getting a feel for the electric trim. In using the electric trim you must first get a feel for how the control button on the control column works. This button sits in the neutral position and you simply push it forward to trim nose-down, or rearward to trim nose-up. However, while the button freely moves in these positions, it is not until you depress the button that the electrical circuit is completed—it is a little trickier than you think, as it commonly takes a lot of downward pressure to make the circuit contact (and activate the motor). It is a good idea to initially monitor the response of the trim motor by gently resting your right hand on the manual trim wheel as the motor is working, thereby getting a sense of the speed at which the motor changes the trim. The trim button also has the undesirable feature of sometimes getting stuck in either the down/forward or down/rearward position, which can quickly be remedied by a thumb flick. But always be ready to overpower the trim motor with two-handed brute force. Should a “runaway” occur, it is crucial that you ignore the column pressure and physically make the nose pitch where you want it to be. Also, be sure to know the location and functioning of the electric trim ON/OFF override.2

To do speed changes well, you must be extremely smooth and slow on any power changes. As with flying any aircraft, smooth and slow power changes will enable you to keep up with the changing pitch condition of the aircraft. If you are jerky (and you don’t want to be a jerk!) and change the throttle settings too fast, the aircraft will attempt to change its attitude in an equally rapid fashion—leaving the pilot “behind the aircraft.” A rapid change in power settings unnecessarily taxes your flying skill. If you perform the throttle changes slowly and smoothly, you will have the time necessary “keep up with the aircraft.” Remember too that aggressive pitch inputs can be very uncomfortable in the twin owing to its higher speeds. At 180 MPH the pilot’s control surfaces have far greater authority than when compared to an aircraft that flies at 90 MPH, owing to the increase in the amount of aerodynamic pressure on those surfaces. If the control column is moved suddenly, not only is it physically uncomfortable for you and your passengers, but there is a risk of over-stressing the airframe.

You will also notice another interesting feature of the twin with respect to speed changes during straight and level cruise flight—owing to the fact that the cruise speed is some 80 MPH faster than what we are normally used to in single-engine trainers, the pitch changes (pitching the nose down) required in a full-range acceleration—for example, from 90 MPH to 180 MPH—will be far greater, and will be made over a far greater period of time. It is not uncommon, for example, for the aircraft to still require pitch-down trim some two minutes after maximum cruise power (24” MP and 2400 RPM) has been set. The same will apply during a speed reduction—such as is done when approaching an airport; after the reduced power has been set, the pilot has to closely monitor pitch for many minutes as the aircraft slowly loses momentum, and the increase in angle of attack has to be gradually increased with fine-tuning of the trim. Overall, get ready to direct a lot more pilot energy to pitch changes when flying a light twin.

During training, and very likely during your flight test, you will be asked to conduct speed changes between 180 MPH and 90 MPH; typically, these will be associated with power changes between 24” MP and 16” MP, respectively.

These speed variations will typically entail the systematic use of flap and gear extension—these provide effective drag control.

When using flaps during a deceleration, bear in mind that just a little flap is amazingly effective in creating that extra drag that gives you that important edge in controlling a speed reduction—without the use of flaps, the deceleration appears very slowly and somewhat improvised. Conversely, the used of flaps makes deceleration controlled and predictable.

Be sure you know the speeds at which the various flap settings can be used.3 Most importantly, however, flap-setting changes will cause considerable pitch changes. As flap settings are increased, get ready to pitch forward to correct the aircraft’s inclination to pitch-up. There is some important advice here—change the flap settings as slowly and as smoothly as possible, thereby allowing your left hand, on the control column, to catch up with the pitch changes that are necessary. All pitch inputs, of course, are made with reference to the altimeter, neutralizing any tendency to climb or descend.

You will see that the flap changes, during a speed increase or decrease, are far more trickier to handle than gear extension or retraction—the movement of the gear only causes what may be described as a “blip” in the aircraft’s pitch configuration. Nevertheless, when the gear exposes itself to the airflow during an extension, you will have to make very subtle changes in pitch to smooth it out.

Finally, throughout speed variations, be sure to target specific speeds—the Instructor or examiner is likely to ask you to change your speed to specific targets—e.g., 90 MPH, with gear extended and 40° flaps. This is a crucial skill during any exercise of precision flying. Speed control is a requisite to stabilized approaches—imperative in larger aircraft—and is fundamental to IFR flying.4

Here is the standard sequence used for a speed reduction, to be applied during training and likely during the flight test. The objective is to slow the aircraft from normal cruise speed—170-180 MPH (typically produced with 24”MP and 2400 RPM), down to 90 MPH, with the gear and flaps extended:

To begin the speed reduction, smoothly reduce the throttle to indicate 16”MP.
As the aircraft slows past 160 MPH, smoothly add 10° flaps.
As the aircraft slows past 150 MPH, extend the gear.
As the aircraft slows below 140 MPH, smoothly add 25° flaps.
As the aircraft slows to the white arch (125 MPH), smoothly add 45° flaps.
As the aircraft approaches 90 MPH, begin to add sufficient power to maintain 90 MPH.5

Throughout this process you are, of course, trimming like mad; your attention must be focused on the altimeter—to determine when pitch inputs are required—and the horizon relative to the glare shield—to determine how much control-column pressures are required.6 Of course you are maintaining a prescribed bugged heading. Smooth—smooth—smooth—this is everything, and smoothness starts with making smooth, slow, and gentle changes in the aircraft’s power and flaps settings.

The reversal of the sequence is simply applied when you are asked to accelerate back to the cruise speed range—namely,

To begin the speed increase, smoothly advance the throttle to indicate 24”MP.
Before the aircraft accelerates out of the white arch (past 125 MPH), smoothly reduce the flaps from 40° to 25° and retract the gear.7
Before the aircraft accelerates past 140 MPH, smoothly reduce the flaps from 25° to 10°.
Before the aircraft accelerates past 160 MPH, smoothly retract the remaining 10° of flaps.

It goes without saying that, throughout this acceleration process you are, of course, trimming like mad, keeping an eagle-eye on the altimeter and the horizon, and on the heading bug, etc.

Safety

Be aware that you must avoid “head-in-the-cockpit” syndrome here, as there is considerable demand for precise control of both flight and engine instruments. To repeat what has been mentioned before in this regard, it is important to establish a safe “division of labour” whereby you do some activity in the cockpit, then stop and scan for traffic—do some more activity, then again stop and look for traffic.

Avoid becoming overly dependent on the Attitude Indicator for pitch reference, and instead develop the habit of using the position the natural horizon relative to the glare shield.8

Since you will be venturing into faster cruise speeds during this exercise, be sure you are aware of the speed limits for turbulent air).

References

1 Such manoeuvres are likely to include 30°-banked turns and climbs and descents

2 Directly in front of the pilot, at the base of the instrument panel below the control column.

3 A flap setting of 10° can be used below 160 MPH, a flap setting of 25° can be used below 140 MPH, and 40° flaps can only be used in the white arch.

4 During an Instrument Rating flight test, the candidate must maintain the aircraft within ±10 knots of “declared speed.” In the Seneca, candidates must maintain 130 MPH during a hold, and 110 MPH during procedure turns and approaches.

5 The power required here will typically be in the range of 17-19”MP.

6 I realize that I am really pushing the limit of reasonable detail here—the point I want to make is that use of the natural horizon is crucial for stable and smooth pitch changes. There is no “nose” visible to the pilot in the Seneca, and therefore pitch changes have to make used the flat horizontal reference provided by the glareshield.

7 Notice with care that the limiting speed for gear retraction is “within the white arch”—or what is less than 125 MPH. If you retract faster than this, damage could occur to the landing gear—it is a very good habit to say “gear speed—gear up” before retraction, and “gear speed—gear down” before extension.

8 Many students starting their multi-engine training have come directly from IFR-simulator training, where the habit of keeping eyes on the Attitude Indicator have become ingrained—this should never be carried over to VFR multi-engine training. There is only one time in which reference is made solely to instruments, and that is when the natural horizon is not adaquately visible.