Note that throttles were first set according to manifold pressure and they were not moved. This procedure affords the simplest and quickest adjustment to cruise power since it involves the fewest control movements. Another advantage Is that by setting equal air flow (rpm, MP, CAT, and CHT) and fuel/air ratio (BMEP drop) on all engines, any discrepancies are in greater evidence and in-flight troubleshooting is facilitated.
it is recommended that blowers be momentarily shifted to the opposite ratio once during every two hours of cruise operation. This procedure is effective in purging the clutch chamber of sludge build-up, which in their advanced stages can render a blower shift impossible. The shock that usually accompanies a blower shift is not harmful to the engine, but may be objectionable to passengers. This shock can be minimized by reducing the pumping load on the supercharger through a manifold pressure reduction of approximately 4 inches while making the shift. No change in rpm is necessary. The use of auto-rich mixture during this procedure is beneficial for several reasons. Most important, the change in fuel/air ratio will alter the temperature pattern within the combustion chamber and cause undesirable deposits to flake off and be exhausted thus prolonging spark plug life.
D. BMEP Fluctuations-13MEP fluctuations associated with lean cruise mixtures (10 to 12 BMEP drop) are attributed to one or a combination of several phenomena characteristic of lean mixture operation depending on the specific circumstances of each individual case.
From the classical curve of BMEP response with mixture strength at fixed throttle and rpm, it should be remembered that at and around best power the manifold pressure response is relatively flat but becomes increasingly steep with leaner mixtures. Thus, while the effect of any fluctuation in engine input may not be visible on the torquemeter at richer mixtures, the effect of this same magnitude of input fluctuation becomes increasingly magnified on the torquemeter with the leaner mixtures. In addition, marginal ignition has a tendency to exacerbate this condition even further as do partially fouled plugs since the leaner mixture may cause partially fouled plugs to misfire, which would otherwise fire normally at the richer mixtures.
To further compound this situation, the cooler combustion temperatures of the leaner cruise mixtures are conducive to certain types of plug deposits not associated with the hotter richer mixtures. It is for this reason that P&W recommends the application of prime for 30 to 60 seconds should severe BMEP fluctuations develop after prolonged lean mixture operation. Under these circumstances when BMEP fluctuations develop with time and are indicative of incipient cold plug fouling, the severe combustion temperature change resulting from the use of prime is beneficial in causing undesirable plug deposits to flake off from thermal shock and be passed harmlessly through the exhaust system.
On the other hand, if severe BMEP fluctuations occur from the start of lean operation, the other engine instruments should be consulted to aid in determining the primary cause of the situation. If all other instruments are normal and steady, this could be an indication of an ignition or spark plug problem that has existed all along but remained unnoticed at the rich mixtures. The previously mentioned bum-out procedure might help if fouled plugs are the cause. Use of an ignition analyzer would also help at this time to check the remaining parts of the ignition system to determine any maintenance action that might be indicated.
Pressure pulsations, peculiar to some carburetor airscoop installations, may cause a fluctuating signal in air metering forces in the carburetor of sufficient magnitude as to be transmitted to the fuel metering section and result in a fluctuating fuel flow which may or may not be picked up by the flowmeter. This engine input phenomenon will not affect torquemeter stability at rich mixtures but will become visible at the leaner mixtures due to the steeper BMEP response at these lean mixtures. Carburetor heat door rigging should be checked for security in this case. In many instances of this nature the following heat door manipulation proved successful with the DC-6 type of installation. Move the carburetor heat lever out of full cold slowly until the CAT gage just starts to increase. Then return the lever back toward cold a small increment to obtain the same CAT that previously existed in full cold, making the last movement of the lever toward the full cold position. In essence, this leaves the heat door open the small amount necessary to bleed higher pressure air from behind the engine into a lower pressure turbulent region immediately downstream of the sharp bend in the induction system. The overall effect is to dampen the pressure pulsations created at the bend making for steadier fuel metering.
In some cases, engine vibration could cause pulsating impact pressure metering through the automatic mixture control that could indicate maintenance action on the automatic mixture control (AMC) unit. In other cases, engine excited vibrations could cause mixture control plate oscillations. In either of these instances any resulting fuel flow fluctuations would not affect torquemeter response at rich mixtures but could very well be visible at lean mixtures. Mixture control rigging should be secure and on occasion it has been found helpful to make the last mixture control adjustment toward the rich position to preset the linkage toward that direction.
It goes without saying that the use of lean mixtures indicates utilization of the so-called long range mixture control plate. This plate results in a flatter fuel flow response with mixture control lever travel in the leaner range.
E. Icing
Keeping in mind the old adage, "Prevention is better than cure," when icing conditions are anticipated, it is desirable to apply preventative carburetor heat rather than risk the possibility of having to employ the more drastic deicing procedures once icing has occurred. A carburetor air temperature of 15 to 20°C is usually sufficient to prevent severe power loss when entering icing condition if applied several minutes prior to entry into those conditions. The automatic mixture control requires three to five minutes to adjust to large temperature changes and may tend to overcompensate for temperatures appreciably above standard. It is desirable, therefore, to enrich the mixture prior to the application of carburetor heat and delay resetting the chart BMEP drop to allow the automatic mixture control time to stabilize. New power settings should then be made for the existing carburetor air temperature. For operators using 25 degrees spark advance in cruise, it should be noted here, that spark must be retarded to the normal 20 degrees position before applying carburetor heat. Again, it should be noted that when this procedure was written, most R-2800s still had the automatic spark advance installed and operational.
When preventative preheat is applied, the maximum carburetor air temperature limit in low blower is 38°C In high blower the maximum CAT is 15°C; however, this limit has been extended to as high as 30°C for some cruise power settings. It is mandatory that these higher CAT limits in high blower, along with the specified BHP, RPM, BMEP, and CHT limits, not be exceeded. If any of these limits are exceeded, the maximum CAT limit reverts to 15°C.
In connection with preheat application, it should be recognized that, with the carburetor heat control in a fixed position, CAT will fluctuate with changes in power, airspeed, cowl flap opening, and particularly changes in the moisture content of the air. It is necessary, therefore, to monitor CAT to assure that sufficient CAT for ice prevention is maintained and that the above mentioned limits are not exceeded.
The first indication of carburetor ice is normally a change in fuel flow and BMEP, which may or may not be accompanied by a decrease in manifold pressure. If ice forms in the air metering elements of the carburetor, a false decrease in air flow will be sensed and the carburetor will reduce fuel metering proportionally to the reduction in air flow indicated by this faulty sense. If this icing occurs during cruise when the mixture is already on the lean side of best power, it has the same effect as further leaning the mixture, thus effecting a further drop in BMEP. Another less common type of carburetor icing may be encountered when descending through a warm moist, region with cold-soaked fuel in the tanks. The fuel, acting as a refrigerant, may cause ice to form in the bleeds between the air chambers of the carburetor, thus increasing the metering suction differential and fuel flow. If the mixture is adjusted to the lean side of best power when bleed ice occurs, BMEP will initially increase. However, if allowed to progress, power will reach a peak and decrease as the mixture enriches further. Throttle ice, screen ice, or any induc-tion ice that restricts air flow would be indicated directly by a loss of manifold pressure and a decrease in fuel flow proportional to the reduction in air flow. This reduced air flow would also be indicated by a loss in BMEP, which in all probability would be the first sign noticed by the pilot.
In the event that carburetor ice occurs, accompanied by a decrease in fuel flow, normal correc-tive action is to: (i) select normal 20 degree (takeoff and climb) spark advance, (11) move the mixture control to auto-rich, and (Iii) apply full carburetor heat for 30 seconds. If icing has been allowed to reach an advanced state where engine power is greatly reduced, the preheat effective-ness of the engine will also be reduced. It may be necessary to apply full preheat for a longer period of time. The carburetor heat control should then be moved slowly toward the cold position and a cheek made of fuel flow and BMEP to assure that the ice has been removed. If determined that the carburetor is free of ice, the CAT should be readjusted to maintain a minimum of 15°C.
Should icing conditions progress far enough to seriously impair engine power, it may be found difficult to obtain enough preheat to deice the carburetor and engine induction system. In this condition the use of continuous primer may be found useful in restoring enough engine power to reestablish a heat source.
When the fuel temperature is known to be well below freezing and bleed-air ice is encountered, as evidenced by an increased fuel flow, the following delcing procedure should be used: Apply carburetor heat to maintain the maximum preheat permissible for that particular power combination. As much as five to fifteen minutes or longer at maximum preheat temperature may be necessary to restore normal operation. If the carburetor has enriched sufficiently to bring about a severe loss in power, the mixture should be manually leaned to restore the desired fuel flow and BMEP. This manual leaning, however, should be practiced only in the cruise or climb power range, with the exception of emergency conditions that may dictate this procedure at higher powers.
After manual leaning fuel flow and BMEP should be closely monitored be cause mixtures will tend to lean out rapidly as the ice is dispelled. With normal operation restored and the ice contributing condition still present, a carburetor air temperature of 15°C should be maintained.
Carburetor alcohol, in the opinion of a few operators, has been helpful in ice elimination, although heat is generally found to be the more effective remedy.
Wing and tail anti-icing for Convair twins is accomplished by routing heated air, taken from the engine augmentor muffs, through ducts in the leading edge structures. Occasionally, in extreme icing conditions, the desired climb and cruise CHT of 200°C has been found to be incapable of adequate heat to the anti -icing system. Under these conditions it is permissible to raise CHT as necessary to a maximum of 232°C provided the following procedure is followed:
1. Engine operation must be confined to normal 20 degree spark advance.
2. Place mixture control in auto-rich.
3. After CAT, CHT, and engine operation have stabilized, it is permissible to manually lean to a maximum of 2 BMEP drop.
F. Descent-If high spark advance was used during cruise, a recommended time for shifting to normal spark is just before starting descent while the mixtures are still leaned. A shift at this point will be evidenced by a positive indication on the BMEP gage whereas a shift during descent is not so apparent, especially if mixtures have been moved to a richer position. In all cases, however, the return to normal spark advance must be accomplished before the final approach and a positive indication of shift should be observed on the BMEP gage.
With pressurized aircraft, the rate of descent is not normally restricted by passenger comfort considerations, and many operators have found it expedient to maintain cruise altitude until relatively close to destination before starting their descent. This procedure permits rapid passage (approximately 2000 feet per minute) through turbulent strata at a conservative airspeed which will impose the least passenger discomfort and likelihood of structural damage from turbulence.
With power combinations of high rpm and low manifold pressure the centrifugal loads of piston movement are no longer sufficiently cushioned by the combustion chamber gas charge and they are transmitted through the link rods to the master rod bearings. These power combinations are known to be detrimental to master rod bearings and if frequently used for prolonged periods of time can eventually lead to bearing distress, thus they should be avoided. In other words this condition is "reverse loading," further described in Chapter 3, under the subheading "Master Rod Bearings." A good rule of thumb for reduced power setting is that a minimum of one inch manifold pressure should be used for each 100 rpm.
Engine stability also dictates that when power is reduced it should be accomplished by a reduc-tion in both manifold pressure and rpm. While it is true that some difficulty may be encountered in maintaining desired cylinder head temperatures during descent at reduced power, these cooler CHTs and the resulting engine instability are largely due to reduced manifold pressure and intake port temperature. It is also true that a reduction in rpm will further cool CHTs, however, it will also facilitate more stable engine operation by allowing more time for the combustion cycle of the slower burning mixture.
It is a safe, although somewhat wasteful procedure, to place mixture controls in the auto-rich position when departing cruise altitude. To effect greater economy, especially on long descents, mixtures may remain manually leaned, however they should be closely monitored to eliminate any tendency toward instability or backfiring which could come about with appreciable changes in altitude, power, and carburetor air temperature. Engine prime may be used to check BMEP drop and mixtures adjusted as required. During a descent with lean mixtures, BMEP and MP must be limited to the maximum cruise values so they also must be monitored and power reduced accordingly.
Blowers may be shifted at any convenient time during descent that power requirement can be met in low blower.
G. Approach -During this period cockpit controls are positioned to prepare the aircraft and engines for the intended landing or possible go-around. If low blower ratio, auto-rich mixtures and takeoff and climb spark advance have not already been selected, they should be at this time. If carburetor heat was used during descent and is not required for a balked landing, it should be removed well in advance of anticipated touchdown to allow time for the automatic mixture control to adjust to the colder temperature and preclude excessive leaning should a go-around at high power be necessary.
it is recommended that approach rpm not be set until the landing gear is extended. This will minimize the detrimental effect associated with operation at high rpm and low manifold pres-sure. When employing this procedure, however, caution must be exercised not to carry low rpm too far into the approach pattern. Should a go-around be necessary or any situation arises where high power is required quickly, the engine is better prepared to provide this power when the rpm has already been advanced.
Whenever the use of full wet takeoff power is anticipated in the event of a rejected landing, ADI should be switched on in sufficient time to allow the system to bleed. At powers less than 1000 bhp or 123 BMEP ADI flow will be negligible and its use should not be postponed beyond the pre-landing check.
H. Landing-Propeller reversing provides more effective deceleration at higher airspeeds and should, therefore, be initiated as soon as possible after the nose wheel is touched down. In manipulating the throttles, it has been found generally desirable to pause momentarily at the reverse idle detent before applying appreciable reverse thrust. This will reduce the yaw tendency that would accompany differing rates of propeller blade actuation and engine power response.
Cowl flaps should be positioned full open as soon as reverse power is applied. The engine baffles, cowling, and CHT instrumentation are designed for fore-to-aft air flow, and are much less effective during reverse pitch operation at low airspeeds.
Propellers should normally be returned to forward.pitch before the airplane has decelerated to 40 knots. Below this speed severe flight control buffeting is usually encountered and exhaust fumes may enter the cabin ventilation system in an objectionable quantity. Nothing like poisoning the passengers with carbon monoxide! Throttles should be returned directly to 1300 rpm and then retarded as required for taxiing. At low airspeeds, reverse propeller wash tends to starve the carburetor scoops, richening the fuel/air ratio at low rpm to the point that backfiring or popping may occur, particularly with throttles in the reverse detent. If desired, positive return to forward pitch may be checked by momentarily depressing the feathering button and observing a decrease in rpm. Correct idle mixture adjustment is of utmost importance to consistent reversing and unreversing performance and also to smooth engine performance during final approach with windmilling propellers and partially closed throttles.
1. Taxi and Shutdown-As previously stated, cowl flaps should be full open for all ground opera-tions, even in cold weather, to prevent excessive temperatures which may not be reflected on CHT gages.
Preference should be given to the 800 to 1000 rpm range for which the optimum idle mixture has been set. This will extend spark plug life considerably. Of course, rpm consistent with genera-tor requirements will take precedence.
Shut down with the mixture control whenever the CHT has dropped to 200'C. It is suggested that throttles are positioned to 1000 rpm and tachometers observed as the mixture control is slowly moved to idle cutoff. A rise of 10 to 20 rpm before dying indicates a correct idle mixture.
If for any reason it is desired to close cowl flaps, this should be delayed until at least 15 minutes after shutdown to allow residual heat to escape. Without this precaution fried ignition harnesses and damaged seals may be the result.