Time for another look at the “Avro” ? 

Prof. Prodyut Das

06/06/2025

With the induction of the of C 235 the veteran “Avro” aka HS 748 will be slowly phased out. The HS 748 is an extremely well- engineered aircraft with an interesting history and considerable possibilities (note 1) not the least of which is the fact that the IAF ones have perhaps as much as 70% of their service life remaining.  More uniquely these genre of aircraft – I have in mind the Fokker F 27 Friendship, our HS 748 and of course the Antonov An 24 had not only rugged structures and “docile” aerodynamics but were also designed to a philosophy that permitted considerable modifications witness the almost grotesque transformation of the AN 24 to the AN 32 with almost double the power and a rear loading ramp. They and the HS 748 have considerable “let” still in their design.

It goes against the grain to waste a very amenable resource so before we send this docile “donkey” to the knacker’s yard let us be absolutely sure we have examined the situation most carefully. Relevant is the government’s willingness to fund the more successful R&D labs, the closer monitoring of projects, the increasing need to develop more new products to shorter time scales. This will create a demand for various test beds for engines, avionics and AWACS related equipment. With their written down book values tending to zero a refurbished Avro would be the ideal platform for such task combining platform docility, reliability, sturdiness and adequate performance for much of the work.. 

It has been reported that HAL proposed in 2024  to the IAF an upgraded HS 748 the reported upgrade consisting of:

1.      A new engine of an unspecified type.

2.      A glass cockpit.

That the IAF was reportedly cold to the suggestion is understandable. The aircraft is being phased out not because of airframe life but because the spares and “rotables” are unavailable – a Godfrey Cold air unit or Water extractor used in the Avro can now possibly be had in an almost new (only slightly dented condition!) only at Stone & Robinson’s of Edgeware Road, reputed scrap merchants established in 1879. The HAL submission thus was a trivial solution. It would give improvement but would not be of any practical use to the customer.

A resubmitted proposal

Perhaps if HAL were to resubmit the proposal as a total systems upgrade addressing the root causes i.e. ease of serviceability and maintainability, and also as a refurbished Technology demonstrator cum air craft upgrade it would overcome the IAF’s objections as well as be presentable as TD aircraft for the DST ( Department of Science and Technology) to develop Civil  Passenger aircraft technologies.

The scope of the project should be enlarged to include the following two categories:

1. Systems replacement

1.      Replacement of the RR Dart 555s by a modern, in production, engine of approximately similar power e.g. PWC 120 or similar.

2.      Replacement of the 4 bladed Dowty Rotol Propellers with multi bladed composite propellers e.g. Hamilton Standard or Hartzell. 

3.      Replacement of all aircraft systems- hydraulic, avionics, electrical, safety, environmental controls etc by current modern systems. To save development time and expense these can be adapted from similar systems used on C 235 or ATR 42s. These are aircraft in the same category and use accessories which would be useful with the usual changes in the circuits and bracketry. This will ensure standardization of supply and spares.

4.      In the past 70 years there has been considerable increase in the efficiency of wheel, brakes and tyres and these need to be incorporated without changing the hardware of the undercarriage mechanism.

The above list is indicative. 

 Role 2; Technology demonstrator  

In addition to the above the HS 748 can be used to develop new technologies.

An example would be:

a.      Electrical environmental control system as in the Boeing 787. The standard air cycle based ECS makes heavy demands on the bleed air requirement which is critical in a  “hot and high” conditions. The proposed system is a hybrid which taps a minimum “for ventilation only” quantity of bleed air, a first estimate of 22 kgs /min flow, expand it through a reciprocator rather than a turbine and the cooled air is then chilled in the usual refrigeration process.

b.     An experimental trial unit to develop “fly by light” for one of the flight controls e.g. the aileron or rudder controls.

c.      Li-Poly batteries and their cooling and ventilation.

d.     Use of composites for entry hatches for the fuselage. Given the modest pressure altitudes of the cabin this would be an ideal case to explore the design.

The challenge in the design – “the Cee gee critter”.

The mere replacement of the engines and propellers with current models will reduce the empty weight of the HS 748 will save about 800 kgs. When other savings e.g. due to switch to 115v AC systems, wheels, tyres, brakes etc are considered along with use of composites for all non-stressed cowlings and hatches the net saving of around 1600 kilos can be expected.

The usual problem with such a major weight improvement is that the CG shifts to the rear (aeronautical old wives’ tale!) and some of the weight gain is lost in ballasting to retain the CG. A first quick figure of 335 kgs ballast  is indicated. 

If precautions are taken during the systems refurbishment from ab initio to  locate systems appropriately, then studies show that, at this level of re-engineering and re-systeming in this case the CG can be tackled so that a zero ballast can be a feasible design target and we get a proven aircraft with almost 18% reduction in empty weight.

A decrease in empty weight in the region of 1.5 tons is anticipated. It would improve the economics and the TO performance – which once caused much harassment early in its carrier- would be a good but achievable performance.

  

In addition to the above TD aircraft there is possibility of Utilizing the Avro airframe with “major” structural redesign of the sub assemblies.

Role 3; Test beds for hybrid helps.

The envisioned test beds would be as test beds for smaller engines including helicopter engines, AWACs antennae development, studies in “Vintokruliya” or “Rotodyne “ layouts. 

This would involve considerable structural modifications and are discussed separately in the notes because they need major structural redesign whereas. The accompanying sketch of a helicopter rotor test bed is provisional and included to indicate the degree of exploitation of the basic design can be in this role. 

A combination of the ALH ROTOR train and engine systems added to the HS748 would get us into the area of hybrid V/STOL which could be very important in generating design data even if the aircraft did not leave the BLR air region. Recall that the very first Kamov Helicopter  with co axial rotor was almost a lash up of a motorcycle engine and the coaxial system.

Role 4

A prototype rear engine twin/ triple turbofan for 40 passengers

The basic idea is to develop a 40 Pax short field rough field using the basic aggregates of the HS 748 for short range commuter but with large capacity but aerodynamically in the same genre and logic of straight wing business Jets such as Yak 40 or the early Cessna Citation. The engines, first estimate of thrust being approximately around 60- 80 kN (2x HTFE 25 or 3x HTFE 25 s?)  and weighing around 10,000 empty can be located in the rear as in conventional small twin or triple rear engines or over the wing as in VFW 614 or the Honda Jets in case of a twin.

In the case- if located at the rear as in the Citation- the CG cannot be compensated by careful design and the fuselage- wing junction will have to be relocated to take care of the CG. This will require remanufacturing but give us a 40 seat short haul jet at very low project cost. The VFW 614 or Honda Jet engines location will need relocation of the tail plane upward and away from the jet efflux.

Summing up.

We have good resources and projects like these can build up crucial design and management leadership manpower at low costs and time -practically by “scrounging” resources. These, rather than ambitious projects that require enormous resources , are a means of developing the manpower. 

Note 1.

The Avro was a design that was probably inspired by the success of the Viscount and starting as a 20- seater in the project studies it grew into a 40 seater and was one of the triad Fokker Friendship, the Handley Page Herald and itself the Avro 748. Built by the traditional “sheet and strip” method it was economical and robust and as the bigger Andover showed it could be modified quite extensively. A factor that helps in this is that the structure is quite modular. The engine was mounted above the wing torsion box and the undercarriage was below – there was no wheel well cutouts resulted in a strong torsion box etc. The 40 seat was the standard configuration and a high-density layout allowed up to 60 seats. Early in it's life the design was AFAIR , capable of being produced in both high and low wing configurations. 

One possible reason why the Avro was selected over the Fokker Friendship in IAF service was it’s traditional construction and materials. The Fokker used “Redux” metal to metal bonding which was duly considered in the selection process though the Fokker’s low loading height was a strong USP. Incidentally The Avro was one of the most successful of British post war projects approximately 380 being produced and sold worldwide, the types simplicity and reliability being much valued.

Its reliability in India was such that the aircraft was rarely in the news barring the occasional accident. It’s start in regular service was less auspicious, there being much lobbying against the type even after it was introduced into service. The objection was raised AFAIR that under certain flight conditions with one engine out the climb out did not conform to certain segments of the requirement. 

The simple solution would have been to withdraw the aircraft from services of those airfields where this rule was being violated and carry on with the rest where this condition was not met but aircraft marketing and counter marketing is like that.

A Dhawan Committee was formed with the instructions to submit a report within eight weeks which was ample. The rigmarole began in full scale with various accusations and counter accusations were made including that of supplying wrong data and finally after three years The S Dhawan Committee reported the shatteringly revealing wisdom that aircraft take off is adversely affected with high runaway temperatures and the MTO has to be appropriately reduced. It affected commercial viability. Now just imagine, 3 years for a decision like that? 

S. Dhawan’s contribution as a leader of our science programmes is there in actual hardware but, only since we cite the US as a SOP in every case including the inappropriate case of funding - I ask you -would the US Government have asked their Werner Von Braun to head a committee to investigate a similar case? 

 The lesson I want to emphasise is 1. Leave aerospace job to the experts and 2. Delay does NOT add value to the investigations.

The irony of our Babu led development programs is perhaps indicated by the selection of this same aircraft, earlier criticized for its poor take off , for the flying rotodome type AWACS which tragically crashed taking a very brave set of scientists with them.The use of the 748 shows the contradictions of our development scenario. Either the earlier objections of marginal power were right and we should not have used the 748 as a platform ; it shows the the near criminal nature lack of basic knowledge of the concerned subject by the leaders  or the accusations of non performance were motivated and the perpetrators should not have gone unpunished or it was a complete callous failure towards prototype security. Something is not right.

I have pleasant memories of the old 748 which used to serve the Pune BLR and BLR- MAS service and in the North one met them in the NDLI- LKO. Sectors. Their construction and riveting were thought provoking and the big windows were a particular pleasure aided by the long slow climb outs and approaches, the flight was at a relatively lower altitude and the smell of burnt Aviation Kerosene was Chanel 7. How wonderful it must have been flying Imperial Airways!